High-mass star-forming cloud G0.38+0.04 in the Galactic Center Dust Ridge contains H2CO and SiO masers
Adam Ginsburg, Andrew Walsh, Christian Henkel, Paul A. Jones, Maria Cunningham, Jens Kauffmann, Thushara Pillai, Elisabeth A.C. Mills, Juergen Ott, J.M. Diederik Kruijssen, Karl M. Menten, Cara Battersby, Jill Rathborne, Yanett Contreras, Steven Longmore, Daniel Walker, Joanne Dawson, John A.P. Lopez
AAstronomy & Astrophysics manuscript no. ms c (cid:13)
ESO 20182018/10/02
High-mass star-forming cloud G0.38+0.04 in the Galacticcenter dust ridge contains H CO and SiO masers Adam Ginsburg , Andrew Walsh , Christian Henkel , , Paul A. Jones , Maria Cunningham , JensKauffmann , Thushara Pillai , Elisabeth A.C. Mills , Juergen Ott , J.M. Diederik Kruijssen , Karl M.Menten , Cara Battersby , Jill Rathborne , Yanett Contreras , Steven Longmore , Daniel Walker , JoanneDawson , , John A.P. Lopez (Affiliations can be found after the references) ABSTRACT
We have discovered a new H CO (formaldehyde) , − , CO maser detected in the Galaxy. Cloud C is one ofonly two sites of confirmed high-mass star formation along the Galactic center ridge, affirming that H CO masers areexclusively associated with high-mass star formation. This discovery led us to search for other masers, among which wefound new SiO vibrationally excited masers, making this the fourth star-forming region in the Galaxy to exhibit SiOmaser emission. Cloud C is also a known source of CH OH Class-II and OH maser emission. There are now two knownregions that contain both SiO and H CO masers in the CMZ, compared to two SiO and six H CO in the Galacticdisk, while there is a relative dearth of H O and CH OH Class-II masers in the CMZ. SiO and H CO masers may bepreferentially excited in the CMZ, perhaps because of higher gas-phase abundances from grain destruction and heating,or alternatively H O and CH OH maser formation may be suppressed in the CMZ. In any case, Cloud C is a newtesting ground for understanding maser excitation conditions.
Key words.
Masers Radio lines: ISM Galaxy: center ISM: clouds ISM: molecules ISM:individual objects: Cloud C
1. Introduction
Masers are important tracers of star formation, shocked gas,evolved stars, and in other galaxies, circumnuclear disks.While many masers are common in the Galaxy and readilydetected in other galaxies (e.g., OH, CH OH, and H O),H CO has only been detected as a maser in seven locationswithin our Galaxy, and so far no instances have been con-firmed in other galaxies (Araya et al. 2007a; Mangum et al.2008) .Most of the H CO masers detected so far have beenobserved as part of dedicated surveys targeting high-massyoung stellar objects (YSOs; Araya et al. 2004, 2007b,2008). Despite concerted effort, very few new masers out-side of Sgr B2 (Whiteoak et al. 1983; Mehringer et al. 1994)have been found since their initial discovery by Forster et al.(1980). All of the known H CO masers are associated withregions of high-mass star formation (Pratap et al. 1994;Araya et al. 2004, 2007b, 2008).The pumping mechanism of the H CO , − , maseris not yet understood. A radio continuum pumping mech-anism was proposed by Boland & de Jong (1981) and latervan der Walt (2014), but the lack of bright radio contin-uum sources near some of the detected H CO maser sources Baan et al. (1986) claimed a maser detection in Arp 220, butMangum et al. (2008) reported that this emission can be ex-plained by thermal processes. However, Baan (private communi-cation) reports that high-resolution observations reveal the emis-sion to be nonthermal. The debate seems unresolved at present. means that this mechanism cannot explain all of the ob-served masers (Mehringer et al. 1994; Araya et al. 2008).van der Walt (2014) ruled out infrared pumping, but sug-gest that collisional pumping may be a viable mechanism.In the van der Walt (2014) framework, high amplifications > are not possible, so additional physical mechanismsmust be in play for the brightest H CO masers.SiO masers are common toward oxygen-rich evolvedstars, namely long period variables (Mira stars) and redsupergiants (see, e.g., Deguchi et al. 2004; Verheyen et al.2012), but extremely rare toward star-forming regions, withonly three known (Zapata et al. 2009b). In the few regionswhere they have been detected - W51 North, Sgr B2 (M),and Orion KL - they closely trace the location of the high-mass YSO, likely pinpointing the base of a high-velocityoutflow (Goddi et al. 2015).Cloud C, G0.38+0.04, is one of the high-column-densityclouds along the central molecular zone (CMZ) dust ridge(Lis et al. 1999; Immer et al. 2012). It is notable for con-taining the brightest 70 µ m point source along that ridgeand the third brightest (after Sgr B2 and Sgr C) alongthe Kruijssen et al. (2015) orbit (Molinari et al. 2011). Itis not detected at 8 µ m with Spitzer (Yusef-Zadeh et al.2009) and is therefore unlikely to be an evolved star, but itis associated with extended 4.5 µ m emission that is gener-ally observed to be associated with molecular (H ) outflows(Chambers et al. 2011). It is among the most centrally con-densed millimeter sources in the CMZ. With a mass in therange 150-2000 M (cid:12) , depending on the assumed tempera- Article number, page 1 of 6 a r X i v : . [ a s t r o - ph . GA ] N ov &A proofs: manuscript no. ms ture, it may contain a single proto-O-star or a proto-cluster(from the SMA-CMZ survey; Battersby, Keto, et al in prep,Walker et al. in prep).In the following Letter, we present the serendipitous de-tection of a H CO maser and corresponding new detectionsof SiO masers in G0.38+0.04.
2. Observations
ATCA observations were performed in 2015 as part of alarger survey of the CMZ. Observations were conducted onApril 2 and 13, May 11, August 12 and 13, and September1 and 4 in the H214, 6A, 1.5C, H75, H75, EW352, and 750Barrays, respectively. The same spectral setup was used foreach array configuration, which included observations of 14spectral lines between approximately 4 and 8 GHz. One ofour main target lines is the , − , transition of H COat 4.82966 GHz. The zoom window at the H CO frequencyyields a channel resolution of 1.9 km s − over a velocityrange 3969 km s − . The sensitivity of the observations was σ = 2 mJy/beam in each 1.9 km s − channel. We assume inthis paper that the absolute positional uncertainty of theobservations is typically 0.4 (cid:48)(cid:48) but no worse than 1.0 (cid:48)(cid:48) , basedon previous ATCA observations (Caswell 2009).
3. Analysis
We detect spatially and spectrally unresolved H CO , − , emission in one narrow line ( σ < . km s − , belowthe instrument resolution) at v = 36 . km s − with anamplitude of 235 mJy/beam, where the restoring beam is . (cid:48)(cid:48) × . (cid:48)(cid:48) . This translates to a brightness temperatureof 1700 K. Molecular emission lines with this brightness aregenerally, not observed in thermally excited regions, so itindicates that there is maser emission.Since the source is spatially and spectrally unresolved,this brightness temperature is a lower limit. If the true emit-ting area is 200 au, such as in the Hoffman et al. (2007) SgrB2 maser spots, the true surface brightness is T B = 10 . K. If the line is narrower than our upper limit of σ < . km s − , it may be even brighter.A literature search revealed that both a Class-II CH OH − A + (6.67 GHz) maser and H O and OH masershave been detected toward Cloud C (Caswell 1998; Argonet al. 2000; Pestalozzi et al. 2005; Caswell 2009; Caswellet al. 2010; Walsh et al. 2011, 2014). We have measuredthe position of the 6.67 GHz CH OH maser from our owndata, and it coincides with the H CO maser in positionto well within the statistical fit errors, much less than theabsolute positional uncertainty ( < . (cid:48)(cid:48) ). There is a watermaser within 1 km s − of the H CO line, and the brightestwater maser is separated by only 4 km s − , so these mayarise from the same region; these H O masers are coincidentwith the H CO masers to within the systematic pointingerrors. The OH and CH OH masers are also within about1 km s − of the H CO line.We searched the Jones et al. (2013) Mopra 7mm surveyof the Class I CH OH − A + (44.069476 GHz) line foremission and found a weak, spatially unresolved line withpeak brightness 0.06 K (0.5 Jy) at the position and veloc-ity of the H CO maser. Assuming the emission comes from < (cid:48)(cid:48) on the sky, as is observed in the H CO line, the truebrightness must be > K, which suggests that this tran-sition is masing. However, Chambers et al. (2011) observed this transition with the EVLA and reported a nondetectionwith a sensitivity of 70 mJy/beam, so further investigationof this line is warranted.We also searched the Jones et al. (2013) data for the SiOv=1 and v=2 J=1-0 lines (43.122079 and 42.820582 GHz).We have clearly detected spatially unresolved emission inboth lines at ∼ km s − at the position of Cloud C. Thedetection of vibrationally excited SiO is a strong indicationthat these are indeed masing transitions. Figure 1 showsthat there is a position offset between the Mopra-detectedSiO and CH OH 44 GHz masers and the ATCA-detectedmasers. This is most likely because the centroid errors fromthe fit to the Mopra moment-0 images are underestimated;there are systematic errors in the Mopra maps (‘streaking’artifacts) that affect sub-resolution centroiding.The SiO masers are offset by ∼ − km s − frommost of the other lines, but their velocities lie within the fullrange of the H O masers. This difference suggests that theH CO and CH OH and some of the H O maser points tracea central protostellar core or disk, while the high-velocityH O and SiO lines may trace part of an outflow or someother structure.Finally, we searched the Jones et al. (2012) Mopra 3mmsurvey for SiO v=1 J=2-1 86.243 GHz emission, but didnot detect any, with a − σ upper limit of 96 mK or 0.89Jy. Given the detection of the 1-0 line at 0.56 Jy, this non-detection is not surprising.The spectral resolution of the Mopra data is 3.6 km s − ,which is close to the FWHM of the measured lines. Giventhe limited signal-to-noise ratio in these data, the lines areconsistent with being spectrally unresolved.Table A.1 shows the measured maser lines toward CloudC, including archival data. Figure 1 shows the maser spotsin position/velocity space. H CO and SiO maser sources To provide context, we summarize the other detected SiOand H CO masers in the Galaxy. The Sgr B2 maser region,the only other one to have both lines detected as masers,shows a velocity offset between SiO and H CO similar tothe offset in Cloud C.
Orion KL : The Orion KL SiO masers are well-studiedwith a long VLBI monitoring program showing that theselines trace the rotating base of an outflow driven by a diskwind (Goddi et al. 2009b; Greenhill et al. 2013). The H Oand SiO masers are closely matched in velocity and gener-ally spatially close: their emission centroid is on the sameposition (Greenhill et al. 2013). No H CO maser emission isseen toward Orion KL; the H CO , − , emission seenthere is thermal with a peak T B ≈ K (Mangum et al.1993).
Sgr B2 (M) : There is only one SiO maser spot in SgrB2, located near Mehringer et al. (1994) H CO Source C(not to be confused with Cloud C, the topic of this paper).The Sgr B2 H CO maser C is peculiar even among theSgr B2 masers in that the emission appears to be spatiallyand spectrally resolved, whereas in other H CO masers inSgr B2, the emission is unresolved. While this might nor-mally hint at thermal emission processes, the high bright-ness temperature ( T B ∼ K) indicates instead thatthere must be multiple unresolved maser spots within thesource. Zapata et al. (2009b) note that the SiO maser isshifted by about 20 km s − from the cloud rest velocity, Article number, page 2 of 6insburg et al: Cloud C Masers(a) (b)
Fig. 1.
Overview of the detected masers colored by velocity. The positional errors on the SiO and CH OH − A + measurementsare much larger than for the other data sets because the measurements are low signal-to-noise from single-dish observations, yetthey are still likely to be underestimated (see Section 3). The gray boxes show the pixel size from the Mopra observations of theselines. (b) is a zoomed-in version of (a) focusing on the interferometer observations. The large X marks the centroid location of theSMA-detected ‘core’ (Walker et al in prep). v SiO = 87 km s − , while v cloud ∼ km s − ; by contrast,the H CO maser is near the cloud velocity or somewhatblueshifted, with v H CO < km s − (Mehringer et al.1994). The remaining Mehringer et al. (1994) H CO maserspots do not have corresponding SiO masers.
W51 North : W51 North is a high-mass YSO that ex-hibits a rich spectrum of NH masers but has no centimetercontinuum source (Henkel et al. 2013; Goddi et al. 2015). Itis detected in SiO at approximately the cloud rest velocity(Zapata et al. 2009b), but is not detected in H CO , − , emission with an upper limit < mJy in a 1 km s − channel(Ginsburg et al in prep). Other H CO sources : The remaining high-mass star-forming regions with H CO maser detections in Araya et al.(2007b) and Araya et al. (2008) do not have known cor-responding SiO masers (G29.96-0.02, NGC 7538, G23.01-0.41, G25.38-0.18, G23.71-0.20, IRAS 18566+0408). How-ever, of these, only NGC 7538 has been searched for SiOmasers (Zapata et al. 2009a). Out of the Araya and Zapatasurveys, which each searched ∼ sources, there were only12 sources common to both samples.
4. Discussion
Out of the now eight known H CO maser-containing regionsin the Galaxy, two are in the CMZ. These two regions,Cloud C and Sgr B2, are the only dense clouds in the CMZwith confirmed ongoing accretion onto a high-mass YSO .Cloud C and Sgr B2 (M) are also the only H CO maser Sgr C also shows some hints of accretion onto high-mass YSOsvia detected outflows (Kendrew et al. 2013) and a 6.67 GHzCH OH maser (Caswell et al. 2010), but it has not yet beensearched for H CO masers. The ultracompact HII regions inSgr B1 and the 20 and 50 km s − clouds appear to be moreevolved (Mills et al. 2011) and may no longer be accreting. CloudE contains a compact molecular core and a 6.67 CH OH GHzmaser (Walker et al in prep, Caswell et al. 2010), but no H COmaser is detected. There is one more 6.67 GHz CH OH masersource south of G0.253+0.016 that may be an isolated site ofhigh-mass star formation. sources with corresponding SiO maser detections and viceversa, though the sample of regions explored in both tracersis small.This high detection rate of masers in star-forming re-gions within the CMZ, despite limited statistical informa-tion, suggests that H CO masers may be an efficient tracerof high-mass star formation in extreme environments. Bycontrast, extensive surveys have shown that the occurrenceof H CO masers in “normal” high-mass star-forming regionsin the Galaxy is very low, < , or 1 of 58 sources in a largesurvey (Araya et al. 2004, 2007b, 2008; Ginsburg et al. 2011,2015a).Given the overall rarity of both SiO masers and H COmasers toward star-forming regions and their apparentprevalence in such regions within the CMZ, is there some-thing different about how high-mass star formation pro-ceeds in the CMZ? Physical conditions on parsec scalesare known to be very different from those in the disk,with greater turbulent velocity dispersion (Shetty et al.2012), higher gas temperatures (Ao et al. 2013; Ginsburget al. 2015b), higher dust temperatures (Battersby et al. inprep), higher pressure (Kruijssen & Longmore 2013), andwidespread emission from shock tracers like (thermal) SiOand HNCO (Jones et al. 2012). However, maser emissioncomes from very small regions (cid:46)
AU, so why shouldthese parsec-scale differences affect the forming stars?One possibility is that these rare masers trace a veryshort period in the lifetime of the forming high-mass YSO.Both masers may trace either an outflow or a disk (Eisneret al. 2002; Goddi et al. 2009a), but the conditions thatallow them to mase may in either case last for a very shorttime. In this scenario, the presence of two such regions inthe CMZ indicates that there is currently an ongoing burstof star formation.Another possibility, which is more closely related to thedriving mechanism of the masers, is that high abundance ofthese species in the CMZ continues from parsec scales downto ∼ au scales. While H CO is abundant throughoutthe ISM and can be produced in the gas phase, its abun-dance is greatly increased when grain surfaces are heated
Article number, page 3 of 6 &A proofs: manuscript no. ms and ices sublimated. SiO is expected to rapidly deplete fromgas into dust in the ISM, but its prevalence throughout theCMZ indicates that there is a great deal of dust process-ing releasing it into the gas phase. CH OH is also preva-lent throughout the CMZ, and the high abundance requiredto produce detectable maser emission implies it is formedon icy grain surfaces and subsequently sublimated, so itspresence is again an indication of grain destruction or heat-ing. The widespread higher gas-phase abundances of thesespecies may allow all high-mass YSOs to go through a phaseof H CO and SiO maser emission in the CMZ, while in “nor-mal” Galactic disk star formation, they cannot.This abundance-based argument would also favor theformation of water masers. However, H O masers are un-derabundant in the CMZ compared to the Galactic disk,though they are present in both Sgr B2 and Cloud C (Walshet al. 2014). Longmore et al. (2013) note that the ratio ofH O masers to thermal NH emission is orders of magni-tude lower in the CMZ than the rest of the Galaxy. Bycontrast, there is a (statistically weak) excess of H CO andSiO masers. If the H O masers come primarily from out-flows, it may be that the greater turbulence in the CMZprevents an adequate path length from being assembled inCMZ gas. Another possibility is that existing H O maserobservations are not sensitive enough, and a populationof lower-luminosity maser sources has so far been missed(Urquhart et al. 2011). Furthermore, the higher pressureand more turbulent CMZ environment means that prestel-lar cores should form with higher densities (Kruijssen et al.2014; Rathborne et al. 2014), which may modify whichmasers are favored.
5. Conclusion
Cloud C in the CMZ dust ridge, a high-mass star-formingregion, is revealed as one of the most maser-rich sites inthe Galaxy. We have reported new detections of H CO , − , OH − A + CO masers toward a consistent set oftarget regions would help determine how unique the asso-ciation between these masers really is.
Acknowledgements.
This work made use of MIRIAD (Sault et al.1995), astropy (Astropy Collaboration et al. 2013), aplpy ( aplpy.readthedocs.org ), pyspeckit (Ginsburg & Mirocha 2011), ds9( ds9.si.edu ), astroquery ( astroquery.readthedocs.org ), splata-logue ( splatalogue.net ) and its member catalogs (Pickett et al.1998; Müller et al. 2005), and the radio-astro-tools toolkit( radio-astro-tools.github.io ). JMDK is funded by a Gliese Fel-lowship. This letter is based on data from ATCA project C3045.
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[email protected] International Centre for Radio Astronomy Research, CurtinUniversity, GPO Box U1987, Perth WA 6845, Australia Max–Planck–Institut für Radioastronomie, Auf dem Hügel69, D–53121 Bonn, Germany Astron. Dept., King Abdulaziz University, P.O. Box 80203,Jeddah 21589, Saudi Arabia School of Physics, University of New South Wales, SydneyNSW 2052, Australia CSIRO Astronomy and Space Science, P.O. Box 76, Epping,NSW 1710, Australia National Radio Astronomy Observatory, Socorro Astronomisches Rechen-Institut, Zentrum für Astronomieder Universität Heidelberg, Mönchhofstraße 12-14, 69120Heidelberg, Germany Harvard-Smithsonian Center for Astrophysics, 60 GardenStreet, Cambridge, MA 02138, USA Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands Astrophysics Research Institute, Liverpool John Moores Uni-versity, IC2, Liverpool Science Park, 146 Brownlow Hill, Liv-erpool L3 5RF, United Kingdom Department of Physics and Astronomy and MQ ResearchCentre in Astronomy, Astrophysics and Astrophotonics,Macquarie University, NSW 2109, Australia
Appendix A: Spectra & Summary Table
We show the extracted spectra in Figure A.1 and the sum-mary of the detected maser lines in Table A.1.
Article number, page 5 of 6 &A proofs: manuscript no. ms
Fig. A.1.
Spectra of each of the measured lines. The Mopra data, both SiO lines and the CH OH − A +
44 GHz Class I line,have spectral resolution 3.6 km s − and channel spacing 1.8 km s − . The ATCA data have spectral resolution 1.9 and 1.4 km s − and beam shapes 4.85 (cid:48)(cid:48) × (cid:48)(cid:48) and 9.4 (cid:48)(cid:48) × (cid:48)(cid:48) for the H CO and CH OH lines, respectively.
Table A.1.
Maser line parameters
Line (cid:96) b σ ( (cid:96) ) σ ( b ) v LSR σ ( v LSR ) Measurement ◦ ◦ (cid:48)(cid:48) (cid:48)(cid:48) km s − km s − CH OH − A + CO , − , OH − A + O G000.375+0.042_A 0.37545 0.04153 1 1 78.8 - Walsh 2014H O G000.376+0.040_A 0.37582 0.03996 1 1 9.5 - Walsh 2014H O G000.376+0.040_B 0.37548 0.03995 1 1 24.4 - Walsh 2014H O G000.376+0.040_C 0.37577 0.03999 1 1 32.3 - Walsh 2014H O G000.376+0.040_D 0.37578 0.03997 1 1 37.3 - Walsh 2014H O G000.376+0.040_E 0.37575 0.03995 1 1 40.4 - Walsh 2014H O G000.376+0.040_F 0.37553 0.03996 1 1 52.5 - Walsh 2014OH 0.37598 0.04014 1 1 36.0 - Caswell 1998Statistical errors on the fit position are given for the single-dish data, and an assumed lower-limit systematic error of 1 (cid:48)(cid:48) is given for each of the interferometric observations.is given for each of the interferometric observations.