FRagmentation and Evolution of dense cores Judged by ALMA (FREJA). I (Overview). Inner ∼ 1000 au structures of prestellar/protostellar cores in Taurus
Kazuki Tokuda, Kakeru Fujishiro, Kengo Tachihara, Tatsuyuki Takashima, Yasuo Fukui, Sarolta Zahorecz, Kazuya Saigo, Tomoaki Matsumoto, Kengo Tomida, Masahiro N. Machida, Shu-ichiro Inutsuka, Philippe André, Akiko Kawamura, Toshikazu Onishi
aa r X i v : . [ a s t r o - ph . GA ] J u l Draft version July 31, 2020
Typeset using L A TEX default style in AASTeX63
FRagmentation and Evolution of dense cores Judged by ALMA (FREJA). I (Overview). Inner ∼ Kazuki Tokuda,
Kakeru Fujishiro, Kengo Tachihara, Tatsuyuki Takashima, Yasuo Fukui,
Sarolta Zahorecz,
Kazuya Saigo, Tomoaki Matsumoto, Kengo Tomida, Masahiro N. Machida, Shu-ichiro Inutsuka, Philippe Andr´e, Akiko Kawamura, and Toshikazu Onishi Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka599-8531, Japan National Astronomical Observatory of Japan, National Institutes of Natural Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan Faculty of Sustainability Studies, Hosei University, Fujimi, Chiyoda-ku, Tokyo 102-8160, Japan Astronomical Institute, Tohoku University, 6-3, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan Laboratoire d’Astrophysique (AIM), CEA, CNRS, Universit´e Paris-Saclay, Universit´e Paris Diderot, Sorbonne Paris Cit´e, 91191Gif-sur-Yvette, France (Received; Revised; Accepted)
Submitted to ApJABSTRACTWe have performed survey-type observations in 1 mm continuum and molecular lines toward densecores (32 prestellar + 7 protostellar) with an average density of & cm − in the Taurus molecularclouds using the Atacama Large Millimeter/submillimeter ArrayAtacama Compact Array (ALMA-ACA) stand-alone mode with an angular resolution of 6 . ′′ ∼
900 au). The primary purpose of thisstudy is to investigate the innermost part of dense cores with view to understanding the initial con-dition of star formation. In the protostellar cores, contributions from protostellar disks dominate theobserved continuum flux with a range of 35%–90% except for the very low-luminosity object. For theprestellar cores, we have successfully confirmed continuum emission from dense gas with a density of & × cm − toward approximately one-third of the targets. Thanks to the lower spatial frequencycoverage with the ACA 7 m array, the detection rate is significantly higher than that of the previoussurveys, which have zero or one continuum-detected sources among a large number of starless sam-ples using the ALMA Main Array. The statistical counting method tells us that the lifetime of theprestellar cores until protostar formation therein approaches the freefall time as the density increases.Among the prestellar cores, at least two targets have possible internal substructures, which are de-tected in continuum emission with the size scale of ∼ Oand N D + ) distributions. These results suggest that small-scale fragmentation/coalescence processesoccur in a region smaller than 0.1 pc, which may determine the final core mass associated with indi-vidual protostar formation before starting the dynamical collapse of the core with central density of ∼ (0.3–1) × cm − . Keywords: stars: formation — stars: protostars — ISM: clouds— ISM: kinematics and dynamics —ISM: individual objects (Taurus) INTRODUCTION
Corresponding author: Kazuki [email protected]
Tokuda et al.
Understanding of protostar formation via the gravitational collapse of a single molecular cloud core (hereafter densecore; e.g., Myers & Benson 1983) is one of the most fundamental issues in astrophysics because the stars are the mini-mum ingredients in galaxies. In addition, protostellar disks as the residual product of the cloud collapse (Terebey et al.1984) eventually form planets (Dutrey et al. 2014). Although a large number of theoretical and observational worksin the last few decades established an overall picture of low-mass star formation (see the review by Shu et al. 1987),many fundamental questions remain to be studied. In particular, understanding the origin of the initial mass function(IMF) is considered to be one of the ultimate goals of star formation studies. The mass spectrum of dense cores mayreflect the mechanism of their formation/fragmentation and should be relevant to the IMF (Goodwin et al. 2008).1.1.
Single-dish Surveys toward Dense Cores in Nearby Star-forming Regions
Single-dish surveys using bolometer cameras revealed that the core mass function (CMF) in the cluster-formingregions of ρ Ophiuchus resembles the stellar IMF, although their observations lack sufficient samples above 0.5 M ⊙ (Motte et al. 1998). The subsequent unbiased survey using a high-density ( ∼ cm − ) molecular gas tracer confirmedsimilar results through their compilation of a much robust number of the sample (Onishi et al. 1996, 1998, 2002;Tachihara et al. 2002). Deep submillimeter continuum maps (e.g., Nutter & Ward-Thompson 2007) found that theCMF turns over at ∼ M ⊙ , and the slope of the low-mass side is also consistent with that of the IMF (see alsoK¨onyves et al. 2015; Marsh et al. 2016). Such large-scale surveys indicate that the core-to-star formation efficiencyis ∼ ∼ cm − in Taurus. Although their tracer, H CO + , possibly traces lower densitygas and affected by the molecular depletion (see Caselli et al. 2002a), the observations of N H + and its deuteratedspecies, which are less sensitive to this problem, also show similar results for some cores (e.g., Tafalla & Hacar 2015;Punanova et al. 2018). Although virial analysis of these objects shows that they are gravitationally bound objects,the presence of such irregular structures imply that environmental effects such as turbulence are still not negligible inthe core dynamics. Numerical simulation by Padoan & Nordlund (2002) claimed that the inner density profile withtwo-dimensional averaging mimics a BE-like structure even if the core has an irregular shape created by turbulentcompression. In any case, the previous single-dish observational studies did not have sufficient angular resolution toresolve the inner ∼ Interferometric Observations of Prestellar Cores
Recent observational studies in the early phase of star formation (i.e., class 0/I phases) suggest that most of the starscan be formed as binary/multiple members (e.g., Chen et al. 2013) whose frequency is ∼ LMA observations of dense cores in Taurus & ρ Ophiuchus (see the recent ALMA survey by Kirk et al. 2017a). However, these studies cannot clarifyhow an isolated dense core evolves into protostar(s) without contaminations from surrounding phenomena (e.g., stellarfeedback). Investigations toward low-mass star-forming complexes, such as Taurus and Bok Globules, are vital to un-derstanding the process of dense core evolution and possible fragmentation leading to a binary/multiple system insidea single core (see also the introduction in Caselli et al. 2019). The fact that the intrinsic column densities of Taurusdense cores are one order of magnitude lower than those in Perseus (Figure 7 in Ward-Thompson et al. 2007) makes itmore difficult for us to find compact/evolved features, which are detectable with interferometers. As one of the mostprominent examples, Dunham et al. (2016) could not find any 3 mm continuum emission inside 56 starless sourcesin Chamaeleon I using the ALMA Main array (12 m array). More recently, Caselli et al. (2019) found a high-density( ∼ cm − ) compact peak toward one of the most well-studied prestellar cores, L1544, in Taurus. The critical nextstep is to reveal universality/diversity regarding the prestellar evolution in a molecular complex, for example, thepresence/absence of fragmentation within a single core and their evolution timescale by the statistical counting.The previous study by Dunham et al. (2016) was a pilot survey to search for extremely high-density objects whosedensities are & cm − , possibly in the FHSC stage. Onishi et al. (2002) implied that the threshold density forthe dynamical collapse of cores is ∼ cm − , and thus investigations of the properties of prestellar cores with adensity of 10 –10 cm − is crucial in order to understand the condition for the onset of star formation. The Jeanslength ( λ J = p πc s /Gρ , where G is the gravitational constant, ρ is the mean density, and c s is the sound speedof 0.2 km s − at 10 K) of 10 cm − gas is ∼ ∼ ′′ at nearby star-forming regions, suchas Taurus, Ophiuchus-North, Lupus, and Chamaeleon, whose distances are ∼
150 pc (e.g., Schlafly et al. 2014). Sucha scale is similar to the beam size of the single-dish telescopes, and the ALMA 12 m array may fail to detect therelatively extended emission due to the filtering-out effect (Onishi 2013). The Atacama Compact Array (ACA, a.k.aMorita Array), which includes short-spacing baselines, is the best tool to explore the innermost part of a dense corewithout a serious missing flux problem compared to the 12 m array alone. We demonstrated the capability of the ACAby discovering a candidate of the prestellar core, possibly leading to brown dwarf or very low-mass star formation(Tokuda et al. 2019). The work was the pilot study of the present project.We present a survey in 1.3 mm continuum and molecular lines toward 39 dense cores (7 protostellar + 32 prestellarcores) in Taurus using the ACA. We name this project as “FRagmentation and Evolution of dense cores Judged byALMA (FREJA),” which is composed of multiple ALMA campaigns (see Sect. 2). Our primary strategy is (1) to findevolved features inside prestellar cores and perform statistical analysis to constrain theories of dense core evolutionusing the ACA stand-alone mode, and (2) to carry out follow-up observations with the 12m array to further characterizethe initial condition of low-mass star formation. In this paper (Paper I), we describe the survey design of this projectwith the ACA stand-alone mode and especially the early results revealed by the 1.3 mm continuum observations.Fujishiro et al. (2020, hereafter Paper II) investigate the nature of a possible FHSC candidate in L1535NE/MC35.
Tokuda et al. OBSERVATIONS AND DATA REDUCTION2.1.
Survey Design and Descriptions of the Observations
Target Selection
We observed a large number of dense cores (32 prestellar core + 7 protostellar cores in total) in Taurus to investigatetheir inner structures with the ACA stand-alone mode in ALMA Cycles 4 and 6. Although our primary targets areprestellar cores, we included protostellar cores associated with young stellar objects (YSOs) for comparison purposes.In the Cycle 4 program (P.I., K. Tachihara, D ∼
150 pc) low-mass, star-forming regions—Taurus, Ophiuchus-North, Chamaeleon, and Lupus—based on the available continuum data obtained by single-dish telescopes (see the description in Tokuda et al. 2019).Because we have confirmed the ∼
50% detection rate in the millimeter continuum emission from prestellar sources, weplanned to observe more objects to search for evolved sources and perform statistical analyses.To obtain a large number that is as unbiased as possible, we targeted the Taurus molecular cloud as the observationregion in the Cycle 6 program (P.I., K. Tachihara, ∼ ′ resolution performed an unbiased survey in CO ( J = 1–0) across the Taurus complex (Mizuno et al. 1995), andthe subsequent C O ( J = 1–0) observations revealed distributions of dense ( ∼ cm − ) filamentary molecular clouds(Onishi et al. 1996, 1998). Based on the large-scale mapping, high-resolution H CO + (1–0) observations using theNobeyama 45 m telescope (Onishi et al. 2002) discovered 44 prestellar cores with a density of & cm − toward high-column density ( > cm − ) regions in Taurus. Such cores detectable in high-density gas tracers are gravitationallybound, and thus they are considered to be prestellar cores (Ward-Thompson et al. 1994, 2007; see also Marsh et al.2014 for the Taurus L1495 region). H CO + sometimes cannot trace the column density peak, possibly due to thedepletion effect onto dust grains in cold/dense environments (e.g., Caselli et al. 2002a). Because this problem happensover a single-dish beam-size-scale even in an optically thin case, the confirmation of another molecular line tracer, suchas a N-bearing species, is used to select evolved prestellar cores robustly. We investigated the N H + detection towardthe Onishi et al. (2002) catalog in the literature (Tatematsu et al. 2004; Tafalla et al. 2004, 2006; Tafalla & Hacar 2015;Punanova et al. 2018) and our independent measurements obtained with the Nobeyama 45 m telescope (Y. Miyamotoet al. in preparation; see also Tokuda et al. 2019). Note that the recent large-scale surveys in the B213/L1495 andB18 regions with the Green Bank Telescope (Seo et al. 2015, 2019; Friesen et al. 2017) also detected NH emissiontoward our selected samples.We selected the observation coordinates to the peak positions of the millimeter/submillimeter continuum emissionobtained by the IRAM 30 m, the JCMT 15 m, and the Herschel telescope (Kauffmann et al. 2008; Palmeirim et al.2013; Marsh et al. 2016; Ward-Thompson et al. 2016; Tokuda et al. 2019). We set the selection criteria in H columndensity as & cm − , which is higher than the column density threshold for prestellar cores (Andr´e et al. 2010). Ifthere are two local peaks in the continuum emission on a single core cataloged by Onishi et al. (2002), we targetedtwo fields to observe each peak. These targets are MC5N/S, MC7N/S, MC13a/W, MC25E/W, and MC33bN/S. Notethat there are two targets (B10 and L1521E; see Hirota et al. 2002; Hacar et al. 2013; Tafalla & Hacar 2015) thatwere not included in the Onishi et al. (2002) catalog. The total number of observed fields is 32 as prestellar sources.We included seven protostellar cores, whose evolutionary stages are mostly class 0/I phases, to investigate differencesbetween pre-/protostellar cores. We previously performed a case study toward the MC27/L1521F class 0 Very Low-Luminosity Object (VeLLO) system using a similar frequency setting (Tokuda et al. 2017, 2018). We include the dataset (hereafter, PROJ-MC27) in this paper. Table 1 and Figure 1 give the properties of the targeted objects and theirpositions on the large-scale Herschel dust continuum image, respectively.2.1.2. Frequency Setting
Millimeter/submillimeter continuum emission is a fundamental tracer of the column density distribution of molecularclouds because the emission is optically thin under the conditions of a typical interstellar medium. According to theALMA sensitivity calculator, Band 6 (211–275 GHz) is one of the most suitable frequencies to maximize the signal-to-noise ratio of thermal dust emission assuming the spectral index, α , is more than 2. For the 7 m array observations inBand 6, the angular resolution of ∼ ′′ , which is a factor of 2 or 3 higher than that of previous single-dish observations,and the ∼ ′′ maximum recovering scale allow us to distinguish whether of not the innermost part has a smoothedcolumn density.In addition to the continuum observations, molecular lines are essential for tracing gas kinematics of the high-density part. In PROJ4, we included two molecular lines, H CO + (3–2) and H CN (3–2), as the dense gas tracers.
LMA observations of dense cores in Taurus Table 1.
Target objectsName RA Dec n ave (H ) a T bk Distance c Stage d Other name Region Project(J2000.0) (J2000.0) (10 cm − ) (K) (pc)MC1 4 h m . s ◦ ′ . ′′ · · · , f L1489 PROJ6MC2 4 h m . s ◦ ′ . ′′ · · · e L1498 PROJ6MC4 4 h m . s ◦ ′ . ′′ · · · e L1495 PROJ6MC5N 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ4,PROJ6MC5S 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ6MC6 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ4MC7N 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ6MC7S 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ6MC8 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ4MC11 4 h m . s ◦ ′ . ′′ · · · L1495 PROJ6B10 4 h m . s ◦ ′ . ′′ · · · · · · L1495 PROJ6MC13W 4 h m . s ◦ ′ . ′′ · · · B213 PROJ6MC13a 4 h m . s ◦ ′ . ′′ · · · B213 PROJ6MC13b 4 h m . s ◦ ′ . ′′ · · · B213 PROJ6MC14N 4 h m . s ◦ ′ . ′′ · · · f B213 PROJ6MC14S 4 h m . s ◦ ′ . ′′ · · · B213 PROJ6MC16E 4 h m . s ◦ ′ . ′′ g B213 PROJ6MC16W 4 h m . s ◦ ′ . ′′ g B213 PROJ6MC19 4 h m . s ◦ ′ . ′′ · · · · · · L1506 PROJ6MC22 4 h m . s ◦ ′ . ′′ · · ·
140 Pre L1521B e L1521 PROJ6MC23 4 h m . s ◦ ′ . ′′ · · · · · · B18 PROJ6MC24 4 h m . s ◦ ′ . ′′ · · · B18 PROJ6MC25E 4 h m . s ◦ ′ . ′′ · · ·
140 Pre B217 e L1521 PROJ6MC25W 4 h m . s ◦ ′ . ′′ · · ·
140 Pre B217 e L1521 PROJ6MC26a 4 h m . s ◦ ′ . ′′ · · · · · ·
140 Class I · · ·
L1521 PROJ6MC27 4 h m . s ◦ ′ . ′′ g L1521 PROJ-MC27L1521E 4 h m . s ◦ ′ . ′′ · · · · · ·
140 Pre · · ·
L1521 PROJ6MC28 4 h m . s ◦ ′ . ′′ · · · B18 PROJ6MC29 4 h m . s ◦ ′ . ′′ · · · B18 PROJ6MC31 4 h m . s ◦ ′ . ′′ e B18 PROJ4,PROJ6MC33bS 4 h m . s ◦ ′ . ′′ h B18 PROJ4,PROJ6MC33bN 4 h m . s ◦ ′ . ′′ h B18 PROJ6MC34 4 h m . s ◦ ′ . ′′ · · · e L1536 PROJ6MC35 4 h m . s ◦ ′ . ′′ i B18 PROJ6MC37 4 h m . s ◦ ′ . ′′ · · · HCL2 PROJ6MC38 4 h m . s ◦ ′ . ′′ · · · · · · HCL2 PROJ6MC39 4 h m . s ◦ ′ . ′′ · · · · · · HCL2 PROJ6MC41 4 h m . s ◦ ′ . ′′ · · · · · · HCL2 PROJ6MC44 4 h m . s ◦ ′ . ′′ e HCL2 PROJ4 a Average number H density estimated from H CO + observations (Onishi et al. 2002). b Kinematic temperature derived fromNH observations (Benson & Myers 1989; Codella et al. 1997; Seo et al. 2015; Feh´er et al. 2016; Friesen et al. 2017). c Distancemeasurements toward individual clouds in Taurus by Galli et al. (2018). Because there is no available data for the L1521region in their study, we applied 140 pc (e.g., Elias 1978) for it. d “Pre” means prestellar cores. The other sources are pro-tostellar cores containing VeLLO (Bourke et al. 2006), class 0/I (Motte & Andr´e 2001), class I (Kenyon et al. 1993a,b), andclass II (Hillenbrand et al. 2012). e − i Benson & Myers (1989), Motte & Andr´e (2001), Codella et al. (1997), Myers et al. (1979),Hogerheijde & Sandell (2000), respectively.
Tokuda et al.
HCL2 B18L1536 L1506 L1489L1498L1521 B213L1495
Figure 1.
Observation targets in the ACA survey on the Herschel/SPIRE 500 µ m continuum map (e.g., Palmeirim et al. 2013).The white crosses denote the central coordinates of the observed field. However, the ACA 7 m array observations could not find significant emission of either line above the 3 σ level towardall the six Taurus targets (see Table 1) even in the three cases (MC5N, MC31, and MC33bN) where the dust emissionwas detected. We thus changed our strategy in PROJ6. The target lines were N D + (3–2), CO (2–1), CO (2–1), and C O (2–1). Although some observational studies toward evolved prestellar cores, such as L183 and L1544,show indications of molecular depletion in N D + at the core center (Pagani et al. 2007; Redaelli et al. 2019), the linebasically correlates well with the dust continuum in cold ( ∼ –10 cm − ) environments.Because the CO isotopologs show highly extended distributions across molecular clouds and often become opticallythick toward dense regions, they are not always suitable as a dense gas tracer. However, recent high-resolutionobservations suggest that they have some benefits in investigating gas properties in an early phase of star formation.For example, if there are embedded protostars or FHSC candidates within the cores, the CO observations cantrace the outflow activity (e.g., Pineda et al. 2011, see also Paper II). The optically thick CO lines also work like athermometer. Tokuda et al. (2018) found warm (15-60 K) CO gas components possibly generated by a turbulent shockin a cold protostellar core, MC27/L1521F, in Taurus. This type of observation may provide us with a new methodto witness a moment of turbulent dissipation at an early phase of star formation (Tachihara et al. 2000; Pon et al.2012, 2014; Larson et al. 2015). The remaining CO isotopologs, CO and C O, fill a gap in the traced density rangebetween N D + and CO. In the MC27/L1521F study (Tokuda et al. 2018), CO and C O traced a peculiar arc-likefeature with a length of ∼ + (3–2) observations (Tokuda et al.2014). Hydrodynamical simulation by Matsumoto et al. (2015) suggested that gravitational torque from multipleobjects promotes such a complex arc-like gas with a size scale of ∼ –10 cm − continuouslyand has the potential to investigate complex gas dynamics related to multiple star formation.2.2. Observations and Data Reductions/Qualities
Detailed data reduction processes and qualities in PROJ4 and PROJ-MC27 were presented in Tokuda et al. (2019)and Tokuda et al. (2018), respectively. We summarize the observation settings and data qualities in Table 2 anddescribe PROJ6 as follows. We performed the ACA (the 7 m array and the TP (Total Power) array) observations
LMA observations of dense cores in Taurus uv range of the 7 marray was 5.4–29 k λ . There were three pointings per target with the same pattern (see also Figure 2). The integrationtime (on-source time) per pointing was ∼ ∼ D + , CO, CO, and C O (Sect. 2.1.2). We used two spectral windows, whose bandwidthand resolution were 2000 MHz and 15.6 MHz, respectively, to obtain continuum images. The central frequencies ofthe two bands were 218.0 and 232.5 GHz. We individually processed the two bands of two targets, MC1 and MC35,following the procedure described in the next paragraph, as a preliminary analysis. After we confirmed that the twobands reproduce almost the same result (see Sect. A.1 in Appendix A), we combined the two bands to enhance theimage sensitivity. The effective frequency of the final processed continuum images is 225.3 GHz ( ∼ tclean task with the multi-scale de-convolver (Kepley et al. 2020) to recover the extended emission. The imaging grid was set to have square pixels of1 . ′′ multi-scale clean are 0, 6, and 18 pixels. The weighting scheme was “Natural.”We manually selected the emission mask (clean box) and continued the deconvolution process until the intensity ofthe residual image reached the ∼ σ noise level. We did not apply the self-calibration process, because the continuumemission of the prestellar sources is quite weak. Note that we applied the same method and parameters to the othertwo different data sets (PROJ4 and PROJ-MC27). Three targets (MC5N, MC31, and MC33bS) were observed in bothPROJ4 and PROJ6 and had confirmed the continuum detection in the two bands, but we used the PROJ6 (1.3 mm)data alone throughout the analyses in this manuscript. We manually selected emission-free pixels from each continuumimage to estimate their rms noise levels. Table 2 summarizes the resultant beam sizes and sensitivities (see also theindividual properties of each target in Table 5 in Appendix A).In the present paper, we normally did not use the TP molecular line data to focus on compact features revealed bythe 7 m array continuum observation alone and to keep the spatial frequency range of both the line and continuumdata similar. Only in Sect. 4.2, we used the combined 7 m and TP array images obtained by the feather task in CASAto further obtain evidence for the realistic substructures within two prestellar cores, MC1 and MC7. Table 2.
Observation settings and qualitiesProject name Code Continuum Molecular lines Beam size Continuum rms Line rms b PROJ4 a CO + , H CN (3–2) 7 . ′′ × . ′′ ∼ − ∼ a CO, CO, C O (2–1), N D + (3–2) 6 . ′′ × . ′′ ∼ − ∼ CO, C O (2–1), N D + (3–2) 7 . ′′ × . ′′ ∼ − ∼ ∼ − . 3. RESULTSFigure 2 shows 1.2/1.3 mm continuum distributions at the innermost part of prestellar/protostellar cores in Taurus.In this section, we describe and characterize the properties of these objects based on the continuum data.3.1.
Prestellar Cores
Figure 2 shows the 7 m array observations in 1.2/1.3 mm continuum toward all of the observed sources. We could notdetect significant ( > σ ) emission in 20 prestellar cores (MC2, MC4, MC6, MC8, MC11, MC13a, MC13W, MC14N,MC16W, MC19, MC22, MC23, MC25E, MC25W, MC29, MC33bN, MC34, MC38, MC44, and L1521E). On thecontrary, we confirmed continuum detection toward 12 prestellar cores (MC1,MC5N, MC5S, MC7N, MC7S, MC16E,MC24, MC31, MC33bS, MC35, MC37, and B10). The detection rate is about one-third of the observed prestellarcores, which is quite high compared to the previous unbiased surveys with a successful detection of zero or one sourcestoward Chamaeleon I and ρ Ophiuchus using the ALMA 12 m array alone (Dunham et al. 2016; Kirk et al. 2017a),
Tokuda et al. (a) Prestellar cores(b) Protostellar cores
NWSW
Figure 2. σ , 6 σ , 9 σ ]. The ellipses in the lower left corner in eachpanel give the angular resolutions. Noise levels and beam sizes of each map are shown in Table 5. Note that the primary beamattenuation is not corrected for the display purpose. (b) Same as (a) but for the protostellar sources. The contour levels are[3 σ , 10 σ , 30 σ , 100 σ ]. because the larger beam and recovering scales in our observations enable us to be more sensitive to the lower-densityregions of the cores (see also discussions in Sect. 4.3). The peak ( F peak ) and total continuum flux ( F ν ) within theobserved field are 2–5 mJy beam − and 5–63 mJy, respectively, as summarized in Table 3. We performed 2D Gaussianfittings to the observed continuum distributions in the image plane after the primary beam correction to derive thecentral coordinate, major/minor axis (FWHM), and position angle. Note that, in the observed field, there is a large LMA observations of dense cores in Taurus M = D F . κ . B ( T d ) (1)where D is the distance to the sources, κ . is the dust opacity at 1.3 mm, and B ( T d ) is the Planck functionat a dust temperature T d . We applied recent distance measurements depending on the region (Galli et al. 2018) and κ . = 0.005 cm g − for prestellar cores (Ossenkopf & Henning 1994; Preibisch et al. 1993). We used gas kinematictemperatures derived from the early NH observations (see Table 1), as T d , assuming that the temperatures of gas anddust are well coupled in the dense regions and a uniform temperature distribution inside the 7 m array observationfields. Table 3 gives the derived mass. The uncertainty of the mass estimation mainly arises from the assumption of κ . due to the grain growth at the central regions of the cores. However, Bracco et al. (2017) reported that thedust emissivity index, β , does not change too much across core radii down to 1000 au at the prestellar cores in theTaurus B213 filament. Although Chac´on-Tanarro et al. (2017) suggested that the opacity can increase by a factor of ∼
2, which is likely traced by ALMA’s resolution, such effect is limited to a small radius, a few hundred astronomicalunits. The grain growth thus does not strongly affect the mass estimation with the 7 m array beam size.The estimated masses ( M obs ) are ∼ − –10 − M ⊙ (see Table 3), indicating that the 7 m array observations likelytrace compact/dense materials at the center of the cores, but show only small fractions of the entire cores themselveswhose total masses are ∼ M ⊙ . We estimated the average H number densities using the following equation: n (H ) = 3 M obs /4 πµ m H R , where µ is the molecular weight per hydrogen (2.8), m H is the H atom mass, and R obs is the observed radius (= √ Major ∗ Minor/2) adopting each distance (Table 1). The derived densities are ∼ (3–10) × cm − (see Table 3). The identification of the high-density part in the molecular clouds based on millimetercontinuum emission alone is sometimes risky, because there may be possible contaminations from unrelated objects,such as distant galaxies (e.g., Tamura et al. 2015; Tokuda et al. 2016). Examining the presence or absence of detectionof high-density gas tracers allows us to confirm whether the continuum emission is indeed arising from the deeplyembedded parts in dense cores. We confirmed C O emission toward all of the PROJ6 targets, and most of thecontinuum peaks have the N D + emission at more than the 3 σ level (see Table 3). We thus conclude that the observed1.3 mm continuum is arising from the cold high-density parts of the cores. Note that we found an additional sourcewith a flux of 19.3 mJy at the edge of the observed field in MC4. The position corresponds to a known YSO candidate,J041412.29+280837.0 (Gutermuth et al. 2009; Rebull et al. 2011).The spatial distributions of the 1.3 mm continuum are diverse: some cores show single peaks while the others containmultiple local peaks. For the first time, we have revealed such a complex substructure toward a large fraction of thesample in Taurus prestellar cores. However, Caselli et al. (2019) cautioned that the interferometric artifacts cause fakesubstructures even if we observe a smoothed distribution. We discuss this possibility in Sect. 4.1 further.3.2. Protostellar Cores
We observed seven protostellar cores (MC13b, MC14S, MC26a, MC27, MC28, MC39, and MC41) with the 7 marray. We detected 1.3 mm continuum emission in all targets. The observed total and peak fluxes are 62–225 mJy and ∼ − , respectively (Table 4), which are about an order of magnitude higher than those in the prestellarsources. Although the early single-dish observations also confirmed this trend (e.g., Motte & Andr´e 2001), the filtering-out effect, due to the interferometric observation, enhanced the intensity contrast between the two evolutionary stages.For example, the early study in MC14S/N provided just a factor of 3 difference in the peak 1.3 mm intensities betweenthe two sources (Bracco et al. 2017). However, our new observations could not detect continuum emission in theprestellar source. We compiled higher-resolution interferometric observations at the same frequency from the literaturefor all protostellar sources (Table 4) to determine the disk contamination. The much longer baseline observations enableus to reveal further compact emission, which is mostly arising from the protostellar disk. Although the fraction ofprotostellar disk contribution ( F disk ) to the total flux covered by the 7 m array depends from source to source, mostof the flux is dominated by F disk with a range of 35–90% except for MC27.MC27 is a unique source in our sample containing a class 0 VeLLO (Bourke et al. 2006; Terebey et al. 2009) in theL1521 region (Figure 1). Note that there is another VeLLO, IRAM 04191–1522 (see Andr´e et al. 1999; Dunham et al.2006), which is located at the outside of the Taurus main cloud. Among the observed protostellar cores, MC27 shows0 Tokuda et al.
Table 3.
Name RA Dec Major Minor P.A. F peak F ν Mass n (H ) Category N D + (J2000.0) (J2000.0) (arcsec.) (arcsec.) (deg.) (mJy beam − ) (mJy) ( × − M ⊙ ) ( × cm − )MC1 04 h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I NMC4 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC5N 04 h m . s ◦ ′ . ′′ h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I · · · MC7N 04 h m . s ◦ ′ . ′′ h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I · · · MC11 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I YB10 04 h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I NMC13a · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC14N · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC16E 04 h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I NMC19 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC22 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC23 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC24 04 h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I YMC25W · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC28NW 04 h m . s ◦ ′ . ′′ · · · a NMC28SW 04 h m . s ◦ ′ . ′′ · · · a NMC29 · · · · · · · · · · · · · · · · · · · · ·
I NMC31 04 h m . s ◦ ′ . ′′ h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I YMC34 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I NMC35 04 h m . s ◦ ′ . ′′ · · · b YMC37 04 h m . s ◦ ′ . ′′ · · · · · · · · · · · · · · · · · · · · · · · · · · · I NMC44 · · · · · · · · · · · · · · · · · · · · · · · · · · · I · · · L1521E · · · · · · · · · · · · · · · · · · · · · · · · · · ·
I N aWe did not categorize the starless peaks in the MC28 protostellar core.bA FHSC candidate (see Paper II). the weakest peak flux, and the contribution from the protostellar disk is ∼ >
50% sensitivity field, and their locations are awayfrom the class I binary source, IRAS 04263+2426 (Chandler et al. 1998; Roccatagliata et al. 2011). The continuumpeaks have the C O emission with a central velocity of ∼ − , which is similar to that of the binary source,indicating that these sources belong to the same system. DISCUSSIONS4.1.
Interpretations of Continuum Detection in Prestellar cores
Revealing the fragmentation/coalescence process of molecular cloud cores just before the onset of star formation iscritical toward understanding the origin of binary/multiple star formation and determining the final stellar mass. For
LMA observations of dense cores in Taurus Table 4.
Name Infrared Source a R.A. Decl. Major Minor P.A. F peak F ν F disk N D + (J2000.0) (J2000.0) (arcsec.) (arcsec.) (deg.) (mJy beam − ) (mJy) (mJy)MC13b IRAS 04166+2706 04 h m . s ◦ ′ . ′′ b YMC14S IRAS 04169+2702 04 h m . s ◦ ′ . ′′ c YMC26a IRAS 04248+2612 04 h m . s ◦ ′ . ′′ · · · NMC27 L1521F-IRS 04 h m . s ◦ ′ . ′′ d YMC28 IRAS 04263+2426 04 h m . s ◦ ′ . ′′ e NMC39 IRAS 04365+2535 04 h m . s ◦ ′ . ′′ f NMC41 IRAS 04369+2539 04 h m . s ◦ ′ . ′′ · · · N aKenyon et al. (1990) for IRAS sources, Bourke et al. (2006) for L1521F-IRS.bCombined Array for Research in Millimeter-wave Astronomy (CARMA) observations with angualr resolution of 1 . ′′ × . ′′
90 byEisner (2012).c Submillimeter Array (SMA) observations with an angular resolution of 1 . ′′ × . ′′
50 by Takakuwa et al. (2018).dALMA observations with angular resolution of 1 . ′′ × . ′′ ′′ by Sheehan, & Eisner (2014). Note that we combined individual fluxesfrom the binary source ( ∼
44 mJy for source N and ∼
40 mJy for source S).f ALMA observations with an angular resolution of 1 . ′′ × . ′′
87 by Aso et al. (2015). example, turbulent perturbations (e.g., Offner et al. 2010) may create a few hundred astronomical unit scale overdenseregions locally. However, some of the magnetohydrodynamic simulations (Matsumoto & Hanawa 2011) suggest thatthe gravitational collapse smoothed out the complex substructures that originated from turbulence. In this case,observations should show that prestellar cores do not have complex substructures (i.e., fragments), possibly leading tomultiple objects. The exploration of such a spatial scale in low-mass dense cores is still incomplete (see Sect. 1.2). Thepresent ACA observations enable us to obtain crucial hints to understanding the mechanism of fragmentation and theevolution of the prestellar collapse phase.The 7 m array measurements obtained indications of multiple local peaks with an intensity of a few mJy beam − toward MC1, MC7N, MC7S, MC16E, MC31, MC33bS, and MC37. The typical separation among the peaks within acore is similar to the present beam size of ∼
900 au. The size scale is much smaller than the Jeans length with a densityof ∼ cm − . If the substructures are real features, the Jeans instability is unlikely to form such fragments. However,we need careful treatments to interpret the substructures obtained by interferometers. Caselli et al. (2019) cautionedthat the incomplete cancellation of Fourier components could produce artificial substructures with a low-level contrastamong the local peaks even if the real core has a constant-density profile at the center.We evaluate the spatial distributions of what the 7 m array observations are looking at the core centers and howrealistic the multiple peaks are, by comparing the synthetic observations using CASA. We generated smoothed coremodels with a Plummer-like function as input models for the simulated observations. The formula of column densityprofile as a function of r , distance from the core center, is as follows: N H ( r ) = N peak (1 + ( r/R flat ) ) p − (2)There are three free parameters: peak H column density ( N peak ), flattening radius ( R flat ), and asymptotic powerindex ( p ). Based on the Herschel/SPIRE measurements (e.g., Marsh et al. 2016) toward the 1.3 mm continuum-detected objects with the 7 m array (see Sect. 4), we adapted a fixed p of 2 and measured N H at a radius of r =6000 au to determine N peak using the equation (2). We prepared three sequences with different N H ( r = 6000 au),maximum = 3 × cm − , average = 1 × cm − , and minimum = 7 × cm − to mimic the observed sample. Foreach column density set, the input R flat are 8000 au, 2500 au, 1500 au, 900 au, and 300 au, which roughly correspondsto the Jeans length (see the definition in Sect 1.2) of 10 cm − , 10 cm − , 3 × cm − , 10 cm − , and 10 cm − gas,respectively. Note that we assumed κ . = 0.005 cm g − and T d = 10 K to convert H column densities to 1.3 mm2 Tokuda et al. continuum fluxes (see also the justifications in Sect. 3.1). In order to investigate the geometric effect of the simulatedprofiles, we considered parametric models with aspect ratios (see Equation (5) in Caselli et al. 2019). We adapted twoaspect ratios of 1.0 and 1.8. The former is called as the fixed model. The latter is a parametric model, which mimicsthe observed aspect ratio of MC31. In the CASA simulator, we used the simobserve task to generate simulatedvisibilities at a central frequency of 225 GHz and a similar integration time of our real observations (see Sect. 2) withthe same 7 m array configuration in Cycle 6. The central sky coordinate was the same as that of MC27, which is closeto the central position of the Taurus main cloud (see Figure 1). We made the simulated images using the tclean taskwith the same parameters that we applied to the real observations (see Sect. 2.2).Figure 3 shows the results of the 7 m array synthetic observations for the cases of average H column density in thefixed (aspect ratio = 1.0) and parametric (aspect ratio = 1.8) models. We could not detect any significant (above 3 σ )emission at the R flat of 8000 au cases. For the other radii models, the fixed models are close to circularly symmetricshapes as a whole, while the parametric models have elongated distributions in the east–west direction. Although theintegrated fluxes, which were measured using the same method we used for our real observations (see Sect. 3), dependon the absolute column density of the input models as shown in Figure 4 (a), the same R flat models well reproducesimilar spatial distributions to each other. We then compared the real and synthetic observations. The case of R flat of2500 au shown in Figure 3 is very similar to the observed substructures, as shown in Figure 2. This means that the 7 marray measurements can artificially produce multiple peaks whose angular separation is similar to the observed beamsize, even if we observe a smooth distribution inside the observed fields (see also in the Appendix A.3). Therefore, wecannot merely claim that the observed substructure is a substantial piece of evidence of fragmentation of prestellarcores. We further discuss the fidelity of the internal substructures by considering the molecular line emission in Sect 4.2. (a) Fixed model (aspect ratio = 1.0)(b) Parametric model (aspect tratio = 1.8) Figure 3.
Synthetic 7 m array observations in 1.3 mm continuum toward smoothed cores (see the text) with different flatteningradii ( R flat ), indicated in the upper-left corner in each panel. The angular resolution, 7 . ′′ × . ′′ σ noise level, where 1 σ is 0.49 mJy beam − . LMA observations of dense cores in Taurus Figure 4. (a) Synthetic 1.3 mm continuum flux of simulated cores with Plummer-like functions. The absolute flux depends onthe column densities, flattening radius, and aspect ratio of simulated cores (see the text). Estimated central densities in unitsof × cm − are denoted at the vicinities of each symbol. The green symbols show the results from parametric models withan aspect ratio of 1.8 (see the text). (b) Observed 1.3 mm continuum flux obtained from the 7 m array observations. Blue andorange histograms show the stacked number of sources detected in prestellar and protostellar cores. Note that we subtractedthe contributions of protostellar disks from the total flux obtained by the 7 m array (see Table 4). Figure 4 (b) shows the histogram of the observed 1.3 mm continuum flux at pre-/protostellar cores. We tentativelyestimated the central volume density using the N flat / R flat of the input models, where N flat is the H column densityat R flat , as shown in each plot in Figure 4 (a). Note that a similar method was also applied to real observations ofcentrally concentrated prestellar cores (e.g., Ward-Thompson et al. 1999). The simulated 1.3 mm flux of the minimumcolumn density model with R flat of 2500 au gives us a detection limit of the central density, ∼ × cm − , which isconsistent with our real observations. Only a few cores (MC5N, MC5S, MC7S, and MC31) have a relatively strongcontinuum flux more than ∼
20 mJy, and their central densities are likely more than 8 × cm − (see also Table 3),which is higher than the other sources. The 1.3 mm fluxes of the bright prestellar sources are comparable to those inprotostellar cores without their disk contributions, as shown in Figure 4 (b). This result can be an indirect piece ofevidence that the bright cores are more evolved than the other weak cores.4.2. Candidates with Internal Substructures in Prestellar Cores
Molecular line observations help us to evaluate the density distribution of dense cores. MC1 and MC7S/N have atleast two significant peaks with an intensity of ∼ − in the 1.3 mm continuum image (Figures 5 (a) and 6(a)), although such a low-intensity contrast distribution can be reproduced by the interferometric artifact as discussedin the previous subsection. The molecular gas tracers (C O, and N D + ) tell us of further fruitful structures (Figures 5and 6).For MC1, the N D + distribution itself indicates that there is a density/size difference between the two continuumsources (Figure 5 (c) and (e)). The southern source has an extended structure, while the size of the northern 1.3 mmpeak is close to the beam with a marginal detection in N D + , indicating that the northern one is much smaller and lessdense than the southern one. Another crucial evidence for the presence of density contrast is that the two continuumpeaks sandwich the C O peak. Although this feature is more apparent in the 7 m array image alone, which filtersout the large-scale emission, the combined 7 m + TP array image also reproduces the same trend (Figures 5 (b) and(d)). If the innermost part of the dense core has a uniform density, but the density is high enough to detect N D + ,CO molecules are considered to be depleted on to dust grains and possibly show a ring-like structure surrounding the4 Tokuda et al. dusty central part (e.g., Caselli et al. 2002b; Crapsi et al. 2005). The present C O distribution in MC1 indicates thatthere is a relatively low-density ( ∼ cm − ) gas in between the overdense continuum peaks. (a) (b) (c)(d) (e) (7m alone) (7m alone) (7m alone)(7m + TP) (7m + TP) Figure 5.
The ACA (7 m + TP array) observations in dust and molecular lines of MC1. (a) Color-scale image and whitecontours show the 1.3 mm continuum emission obtained by the 7 m array alone. The contour levels are the same as Figure 2.The ellipse in each lower-left corner gives the angular resolutions. Color-scale images in panels (b) and (c) show the integratedintensity maps of C O (2–1) and N D + (3–2), respectively, obtained by the 7 m array alone. The contours and levels are thesame as panel (a). (d) and (e) Same as panels (b) and (c) but for the combined image of the 7 m + TP array. Another promising candidate having significant substructures is MC7N/S. The molecular line distribution of C Oand N D + follows a similar manner to that in MC1, as described above (see also Figure 6). The projected separationbetween 1.3 mm peaks in each source is approximately 5000 au, which is significantly larger than the flattening radiusof the synthetic observation, producing fake substructures in the smoothed core (Figure 3). In addition to this, thepositions of each continuum source correspond to local peaks obtained by the early single-dish 0.87 mm continuumobservation (Buckle et al. 2015, see also Figure 11 in Appendix B). We thus exclude the interferometric artifact thatproduces the substructures over a few × Lifetime of 1000 au Scale Compact Structure within the Prestellar Cores in Taurus
We observed the center positions of ∼
30 prestellar cores in Taurus. The observed targets are carefully selected basedon the previous single-dish surveys in the dust continuum and molecular lines. The almost-complete sample allowsus to statistically estimate the lifetime of high-density ( ∼ cm − ) peaks at the center of the core. Onishi et al. LMA observations of dense cores in Taurus (a) (b) (c)(d) (e) (7m alone) (7m alone) (7m alone)(7m + TP) (7m + TP) N S
Figure 6.
Same as Figure 5, but for MC7N/S. (2002) discovered 44 prestellar cores with an average density of & cm − and derived the timescale of protostarformation inside them, as ∼ × yr, assuming that the timescale of one of ∼
100 premain-sequence stars in Tauruswith ages of . yr (Kenyon et al. 1990) is ∼ yr and their constant evolution speed. They conclude that thelifetime of prestellar cores is several times longer than the freefall timescale of gas with 10 cm − . This result isroughly consistent both with both mildly subcritical magnetized cores and with models invoking low levels of turbulentsupport (Ward-Thompson et al. 2007). After their study, several observations additionally found similar density coresin molecular line observations (e.g., Hirota et al. 2002; Hacar et al. 2013; Tafalla & Hacar 2015; Arzoumanian et al.2018) in Taurus, and our survey includes two of them (B10, L1521E). Our recent independent measurements inH CO + , and N H + using the Nobeyama 45 m telescope (K. Tokuda et al., in preparation) tell us that, in total, thereare at most 10 prestellar cores, which are not presented in the Onishi et al. (2002) catalog. Although we can possiblyrevise the total number of prestellar cores in Taurus and their lifetime to be 54, and ∼ × yr, respectively, theprevious conclusion does not change that much.The present 7 m array observations confirmed 1.3 mm dust continuum emission toward 10 sources (see Sect. 3.1).Note that if there are multiple local peaks from single observations (e.g., MC5, MC7, MC33), with at least oneof them detected by the 7 m array, we count it as one source. The total number of continuum-detected objects issignificantly higher than that in the previous surveys, with zero or one successfully detected object by Dunham et al.(2016) and Kirk et al. (2017a). This discrepancy is a quite reasonable result because they targeted much higher density( & cm − ) objects. They evaluated the detectability in continuum emission toward overdense regions within starless6 Tokuda et al. cores using the following equation, Detection > × N total × (cid:16) n Detectable n Limit (cid:17) − (3)where N total is the number of observed cores, n Detectable is the central density threshold for detection, and n Limit isthe observed lower limit for the central number densities of the cores. With N total = 30, n Detectable = ∼ × cm − (Sect. 4.1), and n Limit = ∼ × cm − , Equation 3 tells us that the expected total number of detections is ∼
11, whichis consistent with our present observations.Our study gives us an observational constraint for the lifetime of the innermost parts of prestellar cores on theverge of star formation. Based on our 7 m array 1.2/1.3 mm continuum measurements, we divided the 54 prestellarsources, whose lifetime is ∼ × yr, in Taurus (see the first paragraph in this subsection) into three categories: (I)cores without continuum, (II) cores with weak continuum, and (III) cores with strong continuum (see Table 3). Wetentatively set a flux criterion of 21 mJy, whose density is & × cm − , to separate II and III. The total numbersin categories I, II, and III are 45, 6, and 3, respectively. We assumed that the unobserved cores in this study arecategorized under I, because even the single-dish observations indicate a less evolved feature than our selected target(see the Introduction). If a single core has two peaks with different categories (e.g., MC7N/S and MC33bS/N), wecounted the core as the latter stage. The central density ranges of each category are roughly . × cm − (I),(3–8) × cm − (II), and & × cm − (III). If we adopt the lifetime of prestellar cores, ∼ × yr, the timescaleof each stage can be divided into ∼ × yr (I), ∼ × yr (II), and ∼ × yr (III). We exhibit the estimatedlifetimes in Figure 7, which is similar to the “JWT plot” (after Jessop & Ward-Thompson 2000, see also Figure 2 inWard-Thompson et al. 2007) with a density range of 10 –10 cm − . For the less dense cores (category I), the lifetimeis several or a few times longer than the freefall time of the gas density. This means that supporting forces, suchas the magnetic field and turbulence, still play a role in preventing the freefall collapse. On the other hand, in thefurther high-density regime, the timescale approaches the freefall time (see also K¨onyves et al. 2015). We thus proposethat the density threshold is ∼ cm − when self-gravity becomes the dominant force regulating the core dynamics.Infalling motions may be detectable in the latest stage. However, if the collapsing radius is smaller than the beamsizes of single-dish telescopes, detecting blue asymmetric profile emission is difficult (see the discussion for MC5N, oneof the category III objects, by Tokuda et al. 2019). Interferometric observations with high- J transitions of opticallythick tracers are possible methods to further confirm the internal core dynamics (see also Sect. 6).We briefly mention the lifetime of the possible candidate for the FHSC in MC35. From the dust peak, we founda bipolar outflow, whose dynamical age is less than 10 yr. Paper II described that the observed nature of MC35 tobe consistent with the theoretical properties of the FHSC (e.g., Machida et al. 2008). Finding only one candidate outof the dozen starless targets indicates that the lifetime is less than 10 yr (= 5 × yr * 1/54). This is marginallyconsistent with that from theoretical predictions, ∼ yr (e.g., Larson 1969). We further discussed the lifetime of theFHSC candidate and the comparison between our observations and some numerical simulations in Paper II.4.4. Dense Core Evolution and Its Substructure Formation in the Taurus Molecular Cloud
Most of the dense cores, including our present targets in Taurus, line in filamentary clouds, suggesting that densecore formation is deeply related to the kinematics of filamentary gas, such as fragmentation and collapse as suggestedby the previous studies (Onishi et al. 1998, 2002, see the general review by Andr´e et al. 2014). Our present studyfeatures the subsequent fate after the dense core formation with a ∼ cm − .For category II, some of the parental cores show a highly irregular shape (e.g., MC33, see Onishi et al. 2002;Caselli et al. 2002a; Kauffmann et al. 2008). In this stage, in addition than self-gravity, magnetic field and turbulencemay also play an essential role in forming high-density maxima, which are detectable with interferometers. In additionto this, our measurements discovered at least two promising candidates whose internal substructures have a size scale LMA observations of dense cores in Taurus Density (cm −3 )10 L i f e ti m e ( y r) Onishi et al. 2002Category ICategory II Category IIIfree-fall timefree-fall time × 10
Lifetime vs (central) volume density
Figure 7.
Lifetime vs. (central) volume density. The X-axis error bars show the density range determined by comparisonsbetween the real observations and synthetic ones (see the text in Sect. 4.3). The Y-axis error bar indicates √ N countinguncertainties. Note that the Onishi et al. (2002) data point is the total lifetime that we assume to derive the lifetimes of eachcategory. of ∼ ∼ ∼ − , for a gas temperatureof 10 K, the expected time scale of the coalescence is ∼ × yr (=0.1 pc/0.2 km s − ). This time scale is close to thestatistical lifetime of the Taurus dense core with a density of ∼ cm − (see Sect. 4.3), indicating that coalescence ofdense cores can occur within their lifetime.Regarding fragmentation processes, Nakamura & Hanawa (1997) demonstrated that the core-forming clouds becomeunstable to a bar mode after the magnetically supercritical core formation (see also Machida et al. 2005; Nakamura & Li2002). The two overdense regions in MC7 are distributed along the major axis of the parental core (see Figure 11in Appendix B). Although bar-mode fragmentation is plausible in this case, MC1 shows the opposite trend (see thesingle-dish continuum observation by Motte & Andr´e 2001). These diversities imply that there are several mechanismsthat promote fragmentation/coalescence processes. SUMMARY8
Tokuda et al.
We have carried out a survey-type project toward 32 prestellar and 7 protostellar cores in the Taurus main filamentarycomplex using the ALMA-ACA (Atacama Compact Array, the 7 m + TP array) stand-alone mode with an angularresolution of 6 . ′′ ∼
900 au). Our main conclusions can be summarized as follows:1. A large fraction (30–90%) of the continuum emission from the protostellar cores are contributed by protostellardisks, except in the very low-luminosity protostar case. The continuum observations toward the prestellarsources have revealed the presence/absence of a compact inner structure at the center whose detection rate isapproximately one-third. Thanks to the lower spatial frequency coverage with the 7 m array, the success rate issignificantly higher than the previous ALMA main array surveys. The continuum-detected prestellar cores havea central density, n c , of & × cm − and are more evolved than the remaining sources without continuumdetection (category I, n c . × cm − ).2. Statistical counting of the continuum-detected sources tell us the lifetime of such a high central density object.The subsample of weak continuum-detected sources (category II, ∼ (3–8) × cm − ) shows that its lifetime isslightly longer than the freefall time of the gas density, while the prestellar cores with strong continuum emission(category III, & × cm − ) have a much shorter timescale, which is close to the freefall time. This resultsuggests that the threshold density to dominate the core dynamics by self-gravity during dense core evolution is ∼ cm − .3. Some of the continuum-detected prestellar sources have complex substructures with the size scale of ∼ × O and N D + indicate that there is a real density contrast between each continuum peakin two category II objects, MC1 and MC7. The presence of substructures with a size scale of ∼ ∼ FUTURE PROSPECTSThis work was the first comprehensive dense core survey with the ACA stand-alone mode toward a low-mass star-forming molecular cloud complex and provided further motivations for branching out into several follow-up studies.(1) High-resolution observations using the ALMA main (12 m) array toward some possible candidates just before/afterstar formation (e.g., MC5N, MC35) will elucidate the precise nature of fragmentation and collapse over a few hundredastronomical unit scale or less. (2) The present line setting alone cannot fully explore the gas properties, and thus analternative frequency setup is also needed to understand their kinematics. For example, optically thick tracers with ahigh critical density, such as HCO + and CS, are useful to trace the collapsing motion of dense cores by looking at theirblue asymmetric profile (e.g., Lee et al. 2004). (3) Although we have to consider the distribution of the magnetic fieldto understand the stability of the dense cores, the current ACA capability does not allow us to perform polarizationobservations. Until we get the function, single-dish observations by, e.g., JCMT, IRAM, and SOFIA will play animportant role in obtaining the polarized emission with a relatively low spatial frequency component of dense cores.Extending this type of survey toward other molecular clouds is also crucial to obtain the general picture of starformation. High-density cores just before star formation are quite rare and difficult to find, as statistically demonstratedby the early studies as well as our current project. Nearby ( D ∼
150 pc) low-mass star-forming regions, such asLupus and Ophiuchus-North, are promising candidates to be observed with the ACA. R CrA, ρ Ophiuchus (see also,Kamazaki et al. 2019), and B59 in the Pipe Nebula are also vital targets to compare the properties of dense coresbetween the isolated low-mass star-forming regions and low-to-intermediate cluster-forming sites, and we will eventuallystudy the environmental effects on the dense core evolution.
LMA observations of dense cores in Taurus CO ( J = 1–0) data cube obtained by the FCRAO survey(Goldsmith et al. 2008; Narayanan et al. 2008) to search for CO emission-free positions as reference (OFF) points forthe TP array observations. Software:
CASA (v5.6.0; McMullin et al. 2007), Astropy (Astropy Collaboration et al. 2018), APLpy (v1.1.1:Robitaille & Bressert 2012 APPENDIX A. THE DATA REDUCTION AND IMAGE QUALITIESA.1.
We made continuum images with two individual spectral windows whose central frequencies are 218.0 and 232.5 GHz(see Sect. 2.1.2). As representative examples, we performed the imaging of MC1, which is one of the promisingcandidates with internal substructures (see Sect. 4.1), and MC35, the first core candidate (see Paper II). Figure 8shows that the two frequency images reproduce almost the same result.A.2.
The Image Qualities
Table 5 gives the beam sizes and rms sensitivities of each continuum map. The average angular resolution is 6 . ′′ × . ′′ . ′′
5. We quote this value as the representative angular resolution in the abstract andsummary. A.3.
Comparison between the Real and Synthetic Observations
Figure 9 illustrates the comparison between the real and synthetic observations of MC31 as a representative example.We adopted an aspect ratio of 1.8 and a position angle of 4 ◦ estimated from the real observation (Table 3) as inputparameters of the parametric model. The synthetic core with an R flat of 2500 au and N H ( r = 6000 au) = 1 × cm − reproduces total and peak fluxes similar to the real observation. The residual image does not show large differencesbetween the real and synthetic observations. This result demonstrates that the synthetic model is reasonable to fitthe observed image and that interferometric observations can artificially produce fake substructures even if the corehas a smooth distribution in nature. B. CLUSTERED CORES AT INDIVIDUAL SUBREGIONS IN TAURUSIn the Taurus main filamentary complex (Figure 1), there are several regions with clustering cores. We discuss theirproperties and evolutionary stages by combining early studies and our new measurements.B.1.
The B213 Region
Figures 10 shows 1.2/1.3 mm continuum distributions toward the B213 filamentary cloud. We observed three prestel-lar and two protostellar cores in this region. The line mass of the filament is larger than the critical line mass, M line , crit = 2 c /G (e.g., Stod´olkiewicz 1963; Ostriker 1964; Inutsuka & Miyama 1992), where c s ∼ − is the isothermalsound speed for a gas temperature of ∼
10 K. It means that the filament is unstable against the fragmentation and col-lapse. In this region, several prestellar and prestellar cores are alternate with a separation of ∼ Tokuda et al.
Figure 8. σ noise level ( ∼ − ) of eachmeasurements. The lower-left corners give the angular resolutions. Note that the primary beam attenuation is not correctedfor the display purposes. prestellar sources in the B213 filament (Figure 10 (a)), suggesting that they are not sufficiently centrally concentrated.The onset of dynamical collapse is one option to get that configuration, i.e. detection of continuum emission with the7 m array.Previous studies suggested that a large-scale colliding accretion flow created the B213 filament (Palmeirim et al.2013; Shimajiri et al. 2019). If the filament initially has a uniform density with a 0.1 pc width, which is a quasi-universal value in nearby molecular clouds (Arzoumanian et al. 2011, 2019), we cannot explain such an evolutionarydifference unless there was a density fluctuation in this system at the formation phase. In MC14S and MC13b, thefluctuation of the filament formed overdense regions, and then it might have collapsed into the protostar faster than LMA observations of dense cores in Taurus Table 5.
Beam Properties and Sensitivities in the Continuum ObservationsName B maj (arcsec.) B min (arcsec.) B P . A . (deg.) rms (mJy beam − )MC1 6.8 6.3 -87.9 0.37MC2 6.8 6.1 84.7 0.40MC4 6.9 6.5 78.2 0.39MC5N 6.9 6.5 67.4 0.38MC5S 6.9 6.5 69.3 0.38MC6 7.5 5.2 -54.9 0.55MC7N 6.9 6.5 71.7 0.37MC7S 6.8 6.5 71.2 0.38MC8 7.5 5.2 -54.9 0.65MC11 6.8 6.5 75.8 0.40B10 6.8 6.5 83.6 0.38MC13W 6.8 6.4 83.8 0.36MC13a 6.8 6.4 85.7 0.38MC13b 6.9 6.4 87.7 · · · a MC14N 6.8 6.4 -90.0 0.43MC14S 6.9 6.4 -88.6 · · · a MC16E 6.8 6.4 -81.1 0.33MC16W 6.8 6.4 -78.1 0.41MC19 6.8 6.1 -86.6 0.38MC22 6.8 6.3 -82.4 0.36MC23 6.8 6.1 -86.7 0.35MC24 6.8 6.1 -86.3 0.42MC25E 6.8 6.3 -84.1 0.46MC25W 6.8 6.3 -77.1 0.41MC26a 6.8 6.3 -77.1 0.37MC27 7.2 5.8 -59.6 0.30MC28 6.9 6.1 -83.8 0.44MC29 6.8 6.0 -80.7 0.45MC31 6.8 6.0 -81.2 0.40MC33bS 6.8 6.0 -81.2 0.33MC33bN 6.8 6.0 -80.0 0.45MC34 6.9 5.9 -82.1 0.42MC35 6.8 6.0 -80.8 0.39MC37 6.8 6.2 -77.4 0.35MC38 6.8 6.2 -76.6 0.31MC39 6.9 6.2 -77.6 · · · a MC41 6.8 6.2 -74.9 · · · a MC44 7.3 5.2 -50.1 0.61L1521E 7.0 6.1 -63.3 0.39aThe strong peak intensities make it difficult to accurately measure the sensitivities due to the sidelobe effect. For thesesources, we apply the typical sensitivity, 0.4 mJy beam − , in PROJ6 to draw the contours in Figures 2 and 10. the other cores. In this case, the alternating distribution of pre-/protostellar cores itself is a coincidence rather thanhaving some inherent physical meanings.The present frequency setting allows us to investigate the outflow distribution in CO. Panels (b) and (c) inFigure 10 show the directions of the CO outflows, which are consistent with the early interferometric measurements(Takakuwa et al. 2018, for MC14; Tafalla et al. 2017, for MC13b). The outflow axis seems to be perpendicular to theparental filament elongation, but not in the case of MC14S. This feature means that the large-scale kinematics not only2
Tokuda et al.
Figure 9.
Comparison between the real and synthetic observations in 1.3 mm continuum with the 7 m array. The black ellipsein the lower-left corner gives the observed angular resolution. The left panel shows the real observations of MC31, which isthe same as that in Figure 2. The middle panel is the synthetic observation of the smoothed core model (see the text). Theright panel represents the residual image obtained by subtracting the model image from the observed one. The solid anddotted contours in each panel are the positive and negative 3 σ levels, respectively. The dashed lines indicate where the mosaicsensitivity falls to 50%. determines the rotation axis of the protostellar disk that originated from the filament fragmentation, but also localphenomena within the dense core. Takakuwa et al. (2018) found a counterrotation between the disk and protostellarenvelope in MC14S. They interpreted that the magnetic field may affect the formation of such a complex system.B.2. The L1495 Region
A previous single-dish study found a remarkable filamentary complex in the L1495 region. We observed sevenprestellar cores in Figure 11 (a). In this region, more than 10 dense cores are clustering without any indication ofstar formation. As shown by early molecular line observations in CO and C O (Mizuno et al. 1995; Hacar et al.2013), several filamentary structures are entangled with each other toward this region, indicating that the surroundingfilamentary gas accreted onto the primary dense filaments and then they fragmented into several cores (Tafalla & Hacar2015). Our 7 m array continuum observations found internal structures toward the prestellar cores except for MC6and MC8, suggesting that these cores may be highly evolved and collapse into protostars soon.Tokuda et al. (2019) reported that MC5N has a high-density ( ∼ cm − ) peak at the center of the core, and itis a promising candidate of a brown dwarf prestellar core based on its small parental core mass, ∼ M ⊙ . Wepredicted that another subcore, MC5S, also has a similar density enhancement, if they were formed by a commonmechanism, which is the fragmentation of a filamentary cloud after the radial collapse (Inutsuka & Miyama 1992,1997). The detection of 1.3 mm continuum and N D + in MC5S indicate that there is a high-density peak, and it isconsistent with our prediction. Although the early single-dish study already confirmed the two local peaks of the core(Buckle et al. 2015; Ward-Thompson et al. 2016), we also found 1.3 mm continuum detection with the 7 m array atthe positions of MC7N and MC7S. A crucial difference between MC5 and MC7 is the length of each separation withintheir system; the projected distance of two subcores in MC5 is as long as ∼ ∼ The B18 region
The B18 cloud is at the southern part of the Taurus (Figure 1), and the star formation is more active there thanat the western side, the L1531 region (e.g., Mizuno et al. 1995). Although the overall distribution roughly showsa filamentary structure, the individual dense cores have highly complex morphologies. The separation between thecores is sparse compared to that in the other subregion, e.g., B213/L1495. The recent survey using the Green BankTelescope (Friesen et al. 2017) detected the ammonia emission toward the bright regions in the Herschel dust continuumobservations (Figure 1). There are only three prestellar sources without the 1.3 mm continuum emission from the 7 mobservation. The detection rate is similar to that in L1495.
LMA observations of dense cores in Taurus Taurus B213MC14SMC14NMC13b MC13aMC13W
MC14SMC13b (a) (b)(c)T-tauri
Beam (~24″) −
20 km s -1 −
17 km s -1 -5 − -1 -10 − -1 Figure 10.
Dust continuum and CO outflow distributions toward the Taurus B213 region. (a) The 1.2 mm continuumobtained by the IRAM 30 m/NIKA2 (Bracco et al. 2017). The beam size, ∼ ′′ , is shown by the white circle in the lower-leftcorner. White dotted lines show the field coverage of the 7 m array observations. The black contours are the 7 m array continuumin 1.3 mm with a contour level of 10 σ . (b) and (c) Gray-scale maps show the 7 m array observations in 1.2 mm continuum towardMC14S and MC13b. Red and blue contours show redshifted/blueshifted CO (2–1) emission obtained with the 7 m array. Theintegrated velocity ranges are given at the vicinities of each minimum contour in the figures. The lowest and subsequent contoursteps are 0.4 and 1.2 K km s − , respectively. The angular resolution, 6 . ′′ × . ′′
5, is given by black ellipses in each panel. Thewhite contour shows the 1.2 mm continuum with the contour levels of 10 σ , 30 σ , and 100 σ . In this region, there are some intriguing sources in terms of the early phases of star formation. As mentioned inSect. 3.1, the MC35 is a possible candidate for the FHSC, because the source has no bright infrared sources and theCO observations found a possible compact bipolar outflow. In MC24, we also detect a redshifted high-velocity wing.Because the high-velocity component is not connected to the dust continuum peak, we cannot exclude the possibilitythat the high-velocity component is part of the large-scale outflow from a nearby protostellar source, IRAS 04239+2436(Narayanan et al. 2012). We thus suppose that the probability of a protostellar object being contained in MC24 seemsto be lower than that of MC35.We observed two positions in MC33; one has a dust continuum peak (MC33bS) in the single-dish observation(Kauffmann et al. 2008), and the other shows the N H + , N D + , and NH peak (Caselli et al. 2002a; Crapsi et al.2005). If we accept that the N-bearing species are tracers of chemically evolved regions, the current observationssuggest that MC33bS is a physically evolved peak with a high column density, while MC33bN is a more chemicallyevolved part. Our 7 m array observations also detected 1.3 mm continuum emission in MC33bS, not MC33bN. Thisresult is consistent with the previous studies. MC33 is a vital target for examining the inhomogeneity between a massdistribution and its chemical composition.MC28 is an interesting target harboring both the protostellar source IRAS 04263+2426 and two starless peaks.As mentioned in Sect. 3.2, they share almost the same systemic velocity. This fact indicates that the fragmentationprocess of the parental core or the protostellar feedback produced the dense starless blobs. This system can be anexcellent candidate to study the formation of a wide binary/multiple.B.4. The HCL2 region
The HCL2 (Heiles cloud 2) region (Heiles 1968) has ring-like or filamentary structures observed by dense gas tracersand dust continuum emission (e.g., Onishi et al. 1996; Feh´er et al. 2016, see also Figure 1). Feh´er et al. (2016) estimated4
Tokuda et al. (a) (b) (c)(d)(e)
MC7N MC7SMC6MC8 MC5NMC5SB10 MC7N MC7S MC5NMC5SB10
Figure 11. ∼ σ level. (b–e) Color-scale images and contours show 1.3 mmcontinuum emission observed by the 7 m array. Ellipses in each lower-left corner show the beam sizes. the gas temperature from the NH observations. The Mt. Fuji telescope found a C i peak, whose position is away fromthe dense material, in the eastern part of the region (Maezawa et al. 1999). This result indicates that the region is achemically young region where C i has not yet been fully converted into CO and the evolutionary sequence propagatesfrom the eastern to western side. The absence of the 1.3 mm continuum emission in MC44 in our measurement meansthat the core is in a relatively young phase before the dynamical collapse and follows the global evolutionary sequenceof this region. On the western side, there are several famous star-forming cores, such as L1527 and MC39(=TMC-1A).As shown in Table 4, the F disk / F ν in MC39 is higher than that in other cores harboring class 0/I protostars (MC13b,MC14S). This result indicates that the extended gas in MC39 has been accreted onto the protostar and protostellardisk, which is consistent with the prediction that the protostar is in a late class I phase (Aso et al. 2015).MC37 is a prestellar core with weak continuum emission in our survey, although the complex distribution itself ispossibly arises from the interferometric artifact (see Sect. 4.3).REFERENCES Alves, J. F., Lada, C. J., & Lada, E. A. 2001, Nature, 409,159Alves, J., Lombardi, M., & Lada, C. J. 2007, A&A, 462, L17Andr´e, P., Motte, F., & Bacmann, A. 1999, ApJL, 513, L57Andr´e, P., Men’shchikov, A., Bontemps, S., et al. 2010,A&A, 518, L102 Andr´e, P., Ward-Thompson, D., & Greaves, J. 2012,Science, 337, 69Andr´e, P., Di Francesco, J., Ward-Thompson, D., et al.2014, Protostars and Planets VI, ed. H. Beutheret(Tucson, AZ: Univ. Arizona Press), 27
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