Characterizing the radio continuum nature of sources in the massive star-forming region W75N (B)
A. Rodríguez-Kamenetzky, C. Carrasco-González, J. M. Torrelles, W. H. T. Vlemmings, L. F. Rodríguez, G. Surcis, J. F. Gómez, J. Cantó, C. Goddi, J. S. Kim, S. -W. Kim, N. Añez-López, S. Curiel, H. J. van Langevelde
MMNRAS , 1–12 (0000) Preprint 30 June 2020 Compiled using MNRAS L A TEX style file v3.0
Characterizing the radio continuum nature of sources in the massivestar-forming region W75N (B)
A. Rodríguez-Kamenetzky, (cid:63) C. Carrasco-González, J. M. Torrelles, , W. H. T. Vlemmings, L. F. Rodríguez, G. Surcis, J. F. Gómez, J. Cantó, C. Goddi, , J. S. Kim, S. -W. Kim, N. Añez-López, , S. Curiel and H. J. van Langevelde , Instituto de Astronomía Teórica y Experimental, (IATE-UNC), X5000BGR Córdoba, Argentina Instituto de Radioastronomía y Astrofísica (IRyA-UNAM), 58089 Morelia, México Institut de Ciències de l’Espai (ICE, CSIC), Can Magrans s/n, E-08193, Cerdanyola del Vallès, Spain Institut d’Estudis Espacials de Catalunya (IEEC), E-08034, Barcelona, Spain Department of Earth and Space Sciences, Chalmers University of Technology, SE-43992 Onsala, Sweden INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain Instituto de Astronomía, Universidad Nacional Autónoma de México (UNAM), Apartado Postal 70-264, DF 04510 México ALLEGRO/Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands Korea Astronomy and Space Science Institute, 776 Daedeokdaero, Yuseong, Daejeon 305-348, Republic of Korea Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
30 June 2020
ABSTRACT
The massive star-forming region W75N (B) is thought to host a cluster of massive protostars(VLA 1, VLA 2, and VLA 3) undergoing different evolutionary stages. In this work, wepresent radio continuum data with the highest sensitivity and angular resolution obtained todate in this region, using the VLA-A and covering a wide range of frequencies (4-48 GHz),which allowed us to study the morphology and the nature of the emission of the different radiocontinuum sources. We also performed complementary studies with multi-epoch VLA dataand ALMA archive data at 1.3 mm wavelength. We find that VLA 1 is driving a thermal radiojet at scales of ≈ ≈
130 au), but also shows signs of an incipient hyper-compact HIIregion at scales of (cid:46) (cid:46) ≈ Key words: stars: protostars, massive, mass loss – ISM: HII regions, Herbig-Haro objects,jets and outflows – radio continuum: ISM, stars – radio lines: ISM, stars.
Although it is well-known that the most massive stars have a greatimpact on the galactic environment, many aspects related to theirearly evolutionary stages still remain unknown. For instance, mas-sive protostars are deeply embedded in dense molecular gas, locatedat typical distances of few thousand parsecs. Thus, detailed stud- (cid:63)
Contact e-mail: [email protected] ies of these objects require observations with very high sensitivityand angular resolution. One of the best known massive star-formingregions is W75N (B), located in the Cygnus X complex at a dis-tance of 1.3 kpc (Rygl et al. 2012), comprising dense molecularclouds (Dickel, Dickel & Wilson 1978; Persi, Tapia & Smith 2006)and showing strong maser emission of different molecular species(e.g., Baart et al. 1986; Hunter et al. 1994; Torrelles et al. 1997;Surcis et al. 2009; Krasnov et al. 2015; Colom et al. 2018). Thisregion constitutes an excellent laboratory to study early stages of © 0000 The Authors a r X i v : . [ a s t r o - ph . S R ] J un A. Rodríguez-Kamenetzky massive star formation, since it hosts a cluster of massive protostars(e.g., Shepherd, Testi & Stark 2003), probably undergoing differentevolutionary phases (Torrelles et al. 1997).Since its discovery, W75N(B) has been widely studied, reveal-ing the presence of five radio continuum sources (named VLA 1,VLA 2, VLA 3, VLA 4, and Bc; e.g., Hunter et al. 1994; Tor-relles et al. 1997; Carrasco-González et al. 2015) and a large-scalehigh-velocity molecular outflow (e.g., Davis et al. 1998; Shepherd,Testi & Stark 2003). Among the five radio continuum sources,VLA 1 was proposed to be an evolved young stellar object (YSO),whereas VLA 2 is probably the least evolved YSO in the region(e.g., Torrelles et al. 1997). These two sources are the only ones inthe region that are associated with 22 GHz water (e.g., Torrelles etal. 1997; Surcis et al. 2009, 2011, 2014; Kim et al. 2013) and 6.7GHz methanol maser emission, which was actually detected froma location in between them (e.g., Minier, Booth & Conway 2000;Surcis et al. 2009). Furthermore, polarimetric maser observationsshow the presence of a magnetic field oriented in the direction ofthe molecular outflow (e.g., Hutawarakorn, Cohen & Brebner 2002;Surcis et al. 2009, 2011, 2014). However, despite the deep studiesconducted so far towards W75N(B), the nature of some of the radiocontinuum sources in the region is not well known yet.In this work, we analyze radio continuum data obtained withthe Karl Jansky Very Large Array (VLA) over a wide range offrequencies (4 to 48 GHz), which provide images with the highestsensitivity (rms = 8 µ Jy/beam) and angular resolution (0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09, PA= -69 ◦ ) obtained to date in this region. Part of these datawere presented by Carrasco-González et al. (2015), who focusedtheir attention on the remarkable source VLA 2, reporting throughradio continuum and H O maser observations the transition froman uncollimated outflow to a collimated outflow over a period ofonly 18 years. In this work, we focus on the remaining sourcesin the field: VLA 1, VLA 3, VLA 4, and Bc (see Fig. 1a). Theseobservations allow us to perform a deep multifrequency study ofthe morphology of the sources and of their nature. We also analyzeAtacama Large Millimeter Array (ALMA) 1.3 mm continuum andspectral line archive data obtained toward this region.
The star-forming region W75N (B) was observed with the VLAof the National Radio Astronomy Observatory (NRAO) in its A-configuration at C (6 cm), Ku (2 cm), K (1.3 cm), and Q (7 mm)bands (project code 14A-007). A detailed description of the ob-servations and calibration procedures can be found in Carrasco-González et al. (2015). Deconvolved images were obtained with thetask CLEAN of the Common Astronomy Software Applications(CASA , version 4.1.0) data reduction package, using multifre-quency synthesis (parameter nterms = 2) and multi-scale cleaning(Rau & Cornwell 2011). Primary beam corrections were applied.We split each band data set to build images of narrower bandwidth(1 and 2 GHz), using different weightings, i.e., natural, uniform, andBriggs (Briggs 1995) to achieve the best compromise between sen-sitivity and angular resolution, depending on the analysis performed(e.g., spectral energy distributions, angular size vs. frequency). We NRAO is a facility of the National Science Foundation operated undercooperative agreement by Associated Universities, Inc. https://science.nrao.edu/facilities/vla/data-processing also made a single image combining all four bands (C, Ku, K, and Q;Fig. 1), as well as individual images integrating the full bandwidthof each frequency band (Fig. 2). Moreover, the multifrequency syn-thesis cleaning technique allows us to obtain a spectral index mapcovering the entire range of the observed frequencies.All the radio continuum images presented in this paper, as wellas the spectral energy distribution analysis of the different sourcesare based on the VLA project code 14A-007 (epoch 2014.29). How-ever, in order to study the kinematics of some of the sources in theregion (VLA 4 and Bc) we also reanalyzed previously reported,multi-epoch VLA archive data (project codes AT141, AF381, andAS831; see Carrasco-González et al. 2010, for details on the ob-servations). This, along with our new K-band observations, allowus to compute proper motions in a period spanning 22 years, from1992 to 2014. Calibration of these archive data was undertaken fol-lowing standard VLA procedures, using the Astronomical ImageProcessing System (AIPS ) data reduction package.Parameters of the data sets and images are summarized in Table1 and Table A1, respectively. W75N (B) was observed with ALMA at 1.3 mm during threesessions, on May 6th, 7th, and 11th 2018 (archive ALMA data,project code: 2017.1.01593.S). In total, approximately 16 minuteswere spent on source. During the session on May 7th, only oneminute of useful data on W75N (B) was obtained. The phase centerfor the W75N (B) observations was RA(J2000) = 20:38:37.0 andDec(J2000) = +42:37:51.0, which is ∼
18 arcsec north of the VLA 1-VLA 2-VLA 3 sources. As a result, the mm continuum sourcesdiscussed in this work are detected towards the edge of the ALMAprimary beam (FWHM (cid:39)
27 arcsec). The ALMA images presentedin this paper (Section 3.2) have not been corrected by primary beambut they are of good enough quality for the identification of differ-ent mm continuum sources. The observations were performed usingfour spectral windows (spws). Two spws had 1.875 GHz bandwidthand were centered on 217 .
117 and 230 .
552 GHz. Two further spwshad 117.188 MHz bandwidth and were centered at 216.124 and231.334 GHz. All spws had 1920 spectral channels after hanningsmoothing. During the observations, 46 ALMA telescopes partic-ipated, with a minimum baseline length of 15 m and a maximumbaseline length of 500 m. This resulted in a maximum recoverablescale of ∼ .
5, yielding a synthesized beam size for the1.3 mm continuum observations of 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
86 with a positionangle of − ◦ (Fig. 3).000
86 with a positionangle of − ◦ (Fig. 3).000 , 1–12 (0000) haracterization of W75N (B) Figure 1.
Radio continuum image and spectral index maps of W75N (B) made by the combination of C, Ku, K, and Q bands (epoch 2014.29), usingmultifrequency synthesis cleaning and Briggs weighting (robust 0). (a) Continuum image: contours are –4, 9, 13, 18, 25, 50, 100, and 200 times the rms,8 µ Jy/beam. Panels (b) and (c) show a close-up of the northern region containing sources VLA 1, 2, and 3, and the southern region containing sources Bc andVLA 4, respectively. In both cases, intensity contours of panel (a) are shown over the spectral index map (color scale). The pixels shown in spectral index mapsare those with S/N > 7 in the continuum image. Synthesized beam = 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09 (PA = –69 ◦ ). The most recent data set (epoch 2014.29, see Table A1) provides ra-dio continuum images of W75N (B) with unprecedented sensitivity.Also, these observations enable us to perform a detailed study ofdifferent structures associated with the sources and their emissionnature within several ranges of frequencies.By combining data from all the observed bands (epoch2014.29) we obtain the hitherto deepest (rms (cid:39) µ Jy/beam; beam= 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09, PA = –69 ◦ ) radio continuum image of this region,and a spectral index map (Fig. 1). All the previously known sources in the field (VLA 1, VLA 2, VLA 3, VLA 4, and Bc) are labeled inFig. 1a.In Fig. 2 we show the images of the radio continuum sourcesat C, Ku, K, and Q bands, covering the entire bandwidth in eachcase (see table A1 for the image parameters). Due to spatial filteringeffects that worsen at increasing frequency for a given configuration,C and Ku bands are more sensitive to extended emission than K andQ bands, which tend to favor the detection of compact structures.Taking this into account, and given the wide frequency coverageof our observations, both extended and compact emission of thesources can be studied with high sensitivity. We note that all thesources are detected with a S/N ratio much higher than 5 from C toK band (Figs. 2a,b,c). However, at the highest frequency (Q band in MNRAS , 1–12 (0000)
A. Rodríguez-Kamenetzky
Figure 2.
Radio continuum images of W75N (B) at C (6 GHz), Ku (15 GHz), K (22 GHz), and Q (44 GHz) bands (epoch 2014.29) are shown in panels a, b, c,and d, respectively. White dashed and solid contours represent negative and positive values, respectively, corresponding to different sigma levels: -3, 5, 10, 15,20, 30, 60 times 30 µ Jy/beam (6 GHz image, uniform weighting); –4, 5, 7, 8, 15, 25, 50, 100, 300 times 10 µ Jy/beam (15 GHz image, natural weighting); –4,5, 7, 9, 15, 30, 100, 200, 300, 600 times 10 µ Jy/beam (22 GHz image, natural weighting); –3, 5, 10, 30, 50, 100, 300, 500 times 20 µ Jy/beam (44 GHz image,natural weighting). In each panel, the synthesized beam is indicated with a white ellipse at the bottom left.
Table 1.
PARAMETERS OF THE VLA OBSERVATIONSProject Observation Configuration Central Frequency Flux PhaseDate (GHz) Calibrator CalibratorAT141 a a a b b b b Fig. 2d), only the northern radio sources are detected, with Bc andVLA 4 hardly distinguishable from the noise. The flux densities ofVLA 1, VLA 3, VLA 4, and Bc at each band are listed in Table2 (a detailed discussion of the parameters of VLA 2 is given inCarrasco-González et al. 2015).In addition to all these sources, three new weak ( < µ Jy,see Table 2) compact radio continuum sources are detected in theimages at Ku and K bands, as well as in the image obtained bycombining all four bands. These new sources are located ∼ ∼ ∼ The ALMA continuum observations at 1.3 mm show four cores ina region of ∼
14 arcsec (MM1, MM2, MM3, MM[N]; Fig. 3). Threeof them (MM1, MM2, MM3) have been previously identified byMinh et al. (2010) with the Submillimeter Array (SMA) at 217 and347 GHz, with angular resolution similar to that in our ALMA im-ages. The fourth millimeter core, MM[N], is located ∼ ∼ µ Jy (4 σ in the combination of C+Ku+K+Qbands; Table A1). From Fig. 3 we see that the massive protostarsVLA 1, VLA 2, and VLA 3 are associated with the brightest mil-limeter core, MM1, although the limited angular resolution of theALMA observations (1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
86, PA = − ◦ ) and the north-southdistribution of the sources (with a maximum angular separation MNRAS000
86, PA = − ◦ ) and the north-southdistribution of the sources (with a maximum angular separation MNRAS000 , 1–12 (0000) haracterization of W75N (B) D ec li n a t i on ( J2000 ) Right Ascension (J2000)20 38 37.4 37.2 37.0 36.8 36.6 36.4 36.2 36.0 35.842 37 46444240383634323028 0 20 40 60 D ec li n a t i on ( J2000 ) Right Ascension (J2000)20 38 37.4 37.2 37.0 36.8 36.6 36.4 36.2 36.0 35.842 37 46444240383634323028 10 20 30 D ec li n a t i on ( J2000 ) Right Ascension (J2000)20 38 37.4 37.2 37.0 36.8 36.6 36.4 36.2 36.0 35.842 37 46444240383634323028 6 8 10 12 D ec li n a t i on ( J2000 ) Right Ascension (J2000)20 38 37.4 37.2 37.0 36.8 36.6 36.4 36.2 36.0 35.842 37 46444240383634323028 8 9 10 11 12
SiO(217.1 GHz)Continuum (1.3 mm) SO (216.6 GHz ) CH OH(216.9 GHz) km/skm/skm/smJy/beam
MM1MM3 MM2
VLA 1VLA 2VLA 3Bc[E] VLA[SW]VLA[NE] VLA 4Bc[W]
MM[N] (a) (b)(c) (d) Bd Figure 3. (a) Colour image and contour map of the continuum emission at 1.3 mm. Contour levels are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 × − . Themm sources MM1, MM2, and MM3 are labeled (nomenclature by Minh et al. 2010). A new mm source detected with ALMA ∼ × − km s − . (c) Same as the previous panel but for the CH OH (216.9 GHz) line. Contour levels are 0.1, 1, 2, 3, 4, 5,6,7, 8, 9, 10 × − km s − . (d) Same as the previous panel but for the SO (216.6 GHz) line. Contour levels are 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9,10 × − km s − . Primary beam corrections have not been applied to these images, given that the mm cores are detected at the edge of the ALMAprimary beam (FWHM (cid:39)
27 arcsec; see Section 3.2). Assuming that the ALMA primary beam can be approximated by a Gaussian function, the intensitiesgiven in these images should be corrected by factors of ∼
4, 10, 2, and 1.4 at the positions of MM1, MM2, MM3, and MM[N], respectively. among them of (cid:46)
MNRAS , 1–12 (0000)
A. Rodríguez-Kamenetzky
Because these millimeter continuum cores are found at theedge of the ALMA primary beam and they were only observed at asingle frequency band, we are not able to derive with accuracy theirphysical parameters with the present data. For those estimates werefer to Minh et al. (2010).In Fig. 3 we also show the images of the integrated inten-sity and velocity field (first order momentum) of the molecu-lar lines CH OH [5(1,4)–4(2,2); rest frequency 216.94552 GHz],SO [22(2,20)-22(1,21); 216.64330 GHz], and SiO [v=0 (5-4);217.10498 GHz] as observed with the ALMA archive data. Amain molecular core centered on VLA 1-VLA 2-VLA 3 is de-tected through the CH OH and SO lines. This molecular core, of ∼ ∼ ∼ COwith the SMA ( ∼ − ). Given their angular resolution,these ALMA observations (and the SMA observations; Minh etal. 2010) cannot resolve the structure and motions of the dust andmolecular gas around each of the individual sources VLA 1, VLA2,and VLA 3.On the other hand, the SiO emission shows an irregular dis-tribution covering a broader velocity range (V LSR ≈ OH and SO (Fig. 3), with the highest velocity emission(V LSR ≈ ∼ It can be seen from Figs. 1 and 4 that VLA 1 exhibits a tail-shapedextended component. This is the first time that this particular struc-ture is observed in VLA 1, due to the high sensitivity of our images.In Fig. 1b we show the spatial distribution of spectral indices ( α ,defined as S ν ∝ ν α ) along the source, covering the whole rangeof frequencies. It can be noticed that the central region of VLA 1presents positive spectral indices ( ∼ + α (cid:39) + ◦ ± ◦ ; see Figures 4c and 4d).To study the emission nature of both, the extended and thecompact components of VLA 1, we compute the spectral energydistribution (SED) of the source in the whole range of observedfrequencies with low angular resolution, and the SED at Q band(where most of the extended emission is filtered) with high angularresolution. In Fig. 5 (top panel) we show the SED over the entirerange of frequencies, obtained by measuring flux densities in im-ages with 2 GHz bandwidth, using uniform weighting for C band and natural weighting for Ku, K, and Q bands. All images were con-volved to 0 . (cid:48)(cid:48)
37, corresponding to the lowest resolution in C band.Flux densities were determined by a Gaussian fit within a circularregion of 1.25 arcsec diameter enclosing the source. We note thatthese data were observed with the telescope array same configura-tion, and in this case, the largest scales in the images could be moreheavily filtered at high frequencies, which could result in spuriouslylower values of the spectral index. However, we limited the study ofthe low angular resolution SED to the core of the emission, whichhas a size of ∼
400 milliarcsec (mas). Emission with this size isfully recovered at all bands. Just for description purposes, we haveperformed a fit to the observed flux densities from 4 to 47 GHzthrough an ad hoc function S ν = a ν α [1 − e − b / ν β ] (Fig. 5, top panel).The fit gives a = 0.62, α = 1.38, b = 13.29, and β = 1.42, with S ν in mJy and ν in GHz. Within the uncertainties in the observations,this SED is consistent with an HII region thermal bremsstrahlungspectrum, opaque at low frequencies ( (cid:46)
10 GHz) and optically thinat high frequencies ( (cid:38)
20 GHz). The size of the extended emis-sion, including the tail, is of the order of 1 arcsec, correspondingto an extension of ∼ =
0) images of 1 GHz bandwidth withinthe Q band (see table A1 for image details). All flux densities aremeasured within a circular region of 0.16 arcsec diameter enclos-ing the source. The resulting SED is shown at the bottom panel ofFig. 5. Applying a linear fit to this SED, we obtain a spectral index α = + . ± .
4, consistent with partially optically thick free-freeemission from a thermal radio jet, as predicted by models given byReynolds (1986), and consistent with typical values measured forthermal radio jets (e.g., Anglada, Rodríguez & Carrasco-González2018). This is also in agreement with previous works by Baart et al.(1986) and Torrelles et al. (2003), who detected the presence of aradio jet traced by the distribution of OH and H O masers, respec-tively, along the same direction as the radio continuum emission(PA ≈ +43 ◦ , Torrelles et al. 1997, 2003). According to Reynoldsmodels, the distance to the driving source where the jet becomesoptically thin varies with frequency as a power law. This distance isinterpreted as the angular size of the semi-major axis of the jet, θ ν .Therefore, θ ν ∝ ν − . / (cid:15) , where the index (cid:15) is related to the spectralindex α as α = . − . / (cid:15) in the case of an isothermal jet withconstant velocity and ionization fraction. Thus, from the SED in Qband we derive (cid:15) = + . ± .
5, suggesting a conical jet ( (cid:15) = (cid:219) M , wefollow Equation (3) from Beltrán et al. (2001) based on the modelfrom Reynolds (1986): MNRAS000
5, suggesting a conical jet ( (cid:15) = (cid:219) M , wefollow Equation (3) from Beltrán et al. (2001) based on the modelfrom Reynolds (1986): MNRAS000 , 1–12 (0000) haracterization of W75N (B) Figure 4.
VLA 1 continuum emission (epoch 2014.29). (a) Natural-weighted image at Ku band (central frequency 15 GHz); contours are –3, 6, 8, 15, 20, 25,50, and 100 × µ Jy/beam, the rms of the map. The synthesized beam is 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
16, PA = -75 ◦ . (b) Natural-weighted image at K band (central frequency22 GHz); contours are –3, 6, 7, 9, 13, 15, 20, 30, 50, 100 × the rms = 10 µ Jy/beam. The synthesized beam is 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
10, PA = -74 ◦ . The magenta dottedrectangle indicates the zoomed-in region shown in panel (c). (c) Natural-weighted image at Q band (central frequency 44 GHz); contours are –3, 5, 10, 15, 25,50 × the rms = 20 µ Jy/beam. The synthesized beam is 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
05, PA = -29 ◦ . The magenta dotted rectangle indicates the zoomed-in region shown in panel(d). (d) Uniform-weighted image at Q band; contours are -3, 5, 6, 7, 8, 10, 12, 14 × the rms = 100 µ Jy/beam. The synthesized beam is 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦ . The physical scale of 65 au corresponds to 0.05 arcsec.MNRAS , 1–12 (0000) A. Rodríguez-Kamenetzky
Table 2.
Integrated flux densities of the radio sources detected in epoch 2014.29.Source S C S Ku S K S Q S CKuKQ [mJy] [mJy] [mJy] [mJy] [mJy]VLA 1 3.8 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± < ± ± ± ± < ± < ± ± < ± < ± ± < ± < < ± < ± σ . Errors are computed as the quadratic sum of bothcalibration and fitting error, except in the case of the last column, S CKuKQ , which correspond tofitting errors. See Table A1 for image details. (cid:219) M − M (cid:12) yr − = . (cid:20) ( − α )( α + . ) . − α (cid:21) . × (cid:20)(cid:18) S ν mJy (cid:19) (cid:16) ν
10 GHz (cid:17) − α (cid:21) . (cid:18) d kpc (cid:19) . (cid:16) ν t
10 GHz (cid:17) . α − . × (cid:18) θ o rad (cid:19) . (cid:18) T e K (cid:19) − . ( sin i ) − . (cid:18) V jet
200 km s − (cid:19) (cid:18) x o (cid:19) , (1)where S ν is the flux density at the frequency ν , α the spectralindex, θ o = .
88 rad the jet injection opening angle, estimated as θ o = ( θ min / θ maj ), where θ min and θ maj are the deconvolvedminor and major axes of the Gaussian fit to the source respectively.T e = 10 K is the electron temperature, x o the ionization fraction,V jet the jet velocity, i the jet inclination angle, ν t the turn-overfrequency, and d = 1.3 kpc the distance to the region W75N (B).The values of ν and S ν correspond to the Q-band image, being ν =44 GHz the central frequency of the band and S ν = 15.7 ± α = +0.5 ± ν = 47 GHz. Thus, wetake this value as a lower limit to the turn-over frequency ν t (abovewhich the entire jet becomes optically thin). Since we cannot specifythe jet inclination angle i , we adopt i = ◦ , as variations from 45 ◦ to90 ◦ only change the mass-loss estimate by less than 10%. Moreover,both the ionization fraction and the jet velocity are unknown. Typicalvalues for V jet range from 100 to 1000 km s − , while the ionizationfraction is usually assumed to be 10% (e.g., Anglada, Rodríguez& Carrasco-González 2018) for low-mass protostars. However, thisvalue is very uncertain, and could probably be higher for high massprotostars. According to this, we estimate lower and upper limitsfor the mass loss rate of ∼ × − M (cid:12) yr − (assuming x o = 1 andV jet = 100 km s − ) and ∼ × − M (cid:12) yr − (assuming x o = 0.1and V jet = 1000 km s − ), respectively.On the other hand, we can obtain some estimates of the physicalparameters of the HII region using data from C to K bands. Derivedparameters are presented in Table 3. Within the Rayleigh-Jeansregime, the brightness temperature T B can be written in terms ofthe flux density S ν at frequency ν , and the solid angle subtendedby the source Ω S , as T B = S ν c k ν Ω S . The solid angle Ω S of theelliptical Gaussian fitted to the source brightness profile is calculatedas ( π / ) × FWHM maj × FWHM min . At the central frequencyof each band, T B is always small compared with the electronic temperature T e , assumed to be of the order of 10 K for an HII region(since this is the temperature at which hydrogen ionizes). Knowingthe brightness temperature, the optical depth τ ν can be calculatedfrom T B = T e ( − e − τ ν ) . As the HII region is more optically thinat the K band, we choose this band to estimate some parameters.Assuming the ionization number equals the recombination number,we can estimate the ionizing photon rate (cid:219) N, i.e. the number ofionizing photons λ <
912 Å per unit of time, necessary to accountfor the emission observed at K band: (cid:219) N = π (cid:18) L (cid:19) n e n p η, (2)where η = × − cm s − the "case B" recombination coeffi-cient (i.e., the number of recombinations per unit time, volume,and electron and ion density) to levels (cid:62) e ≈ K; n e and n p are, respectively, the number density of electrons and pro-tons (assumed to be the same), and L is the characteristic size ofthe region (estimated as the geometric mean of the source majorand minor axes). Assuming a homogeneous, spherical HII regionof depth L, n e can be expressed in terms of the emission mea-sure (EM) and the geometrical depth as n e = ( EM / L ) / (withEM = ∫ L0 n e n p dl which, in the case of a homogeneous ionized hy-drogen medium of depth L approximates to E M (cid:39) n e L ). In turn,EM can be derived from the expression of the optical depth asEM [ cm − pc ] = . τ ν [ T e / K ] . [ ν / GHz ] . . Thus, computingS ν , Ω , T B , τ ν , L, n e , and EM, at the central frequency of band K(Table 3), we finally obtain an ionizing photon rate (cid:219) N ≈ × photons s − . This value is much lower than typical estimations forO-B stars ( ≈ photons s − ). Moreover, characteristic values ofemission measure and electron number density for HCHII regionsare EM (cid:38) pc cm − and n e (cid:38) cm − , respectively (Kurtz2005). In the case of VLA 1, the electron number density we obtainis of the same order of typical values for HCHII regions, while theemission measure is about two orders of magnitude lower (see Table3). Mass-loss rates in the range of 10 − M (cid:12) yr − (for low-mass YSOs) to 10 − M (cid:12) yr − (for high-mass YSOs) have beendetermined in the literature (see Anglada, Rodríguez & Carrasco-González 2018, and references therein). Thus, our estimates forthe mass-loss rate of VLA 1, together with the physical parame-ters we obtain for the HII region, lead us to conclude that VLA 1might be a massive protostar driving a thermal radio jet, whichseems to be at the very beginning of the photoionization stage. Thepresence of a radio jet coexisting with an UCHII has been also re- MNRAS000
912 Å per unit of time, necessary to accountfor the emission observed at K band: (cid:219) N = π (cid:18) L (cid:19) n e n p η, (2)where η = × − cm s − the "case B" recombination coeffi-cient (i.e., the number of recombinations per unit time, volume,and electron and ion density) to levels (cid:62) e ≈ K; n e and n p are, respectively, the number density of electrons and pro-tons (assumed to be the same), and L is the characteristic size ofthe region (estimated as the geometric mean of the source majorand minor axes). Assuming a homogeneous, spherical HII regionof depth L, n e can be expressed in terms of the emission mea-sure (EM) and the geometrical depth as n e = ( EM / L ) / (withEM = ∫ L0 n e n p dl which, in the case of a homogeneous ionized hy-drogen medium of depth L approximates to E M (cid:39) n e L ). In turn,EM can be derived from the expression of the optical depth asEM [ cm − pc ] = . τ ν [ T e / K ] . [ ν / GHz ] . . Thus, computingS ν , Ω , T B , τ ν , L, n e , and EM, at the central frequency of band K(Table 3), we finally obtain an ionizing photon rate (cid:219) N ≈ × photons s − . This value is much lower than typical estimations forO-B stars ( ≈ photons s − ). Moreover, characteristic values ofemission measure and electron number density for HCHII regionsare EM (cid:38) pc cm − and n e (cid:38) cm − , respectively (Kurtz2005). In the case of VLA 1, the electron number density we obtainis of the same order of typical values for HCHII regions, while theemission measure is about two orders of magnitude lower (see Table3). Mass-loss rates in the range of 10 − M (cid:12) yr − (for low-mass YSOs) to 10 − M (cid:12) yr − (for high-mass YSOs) have beendetermined in the literature (see Anglada, Rodríguez & Carrasco-González 2018, and references therein). Thus, our estimates forthe mass-loss rate of VLA 1, together with the physical parame-ters we obtain for the HII region, lead us to conclude that VLA 1might be a massive protostar driving a thermal radio jet, whichseems to be at the very beginning of the photoionization stage. Thepresence of a radio jet coexisting with an UCHII has been also re- MNRAS000 , 1–12 (0000) haracterization of W75N (B) Figure 5.
VLA 1 spectral energy distribution (epoch 2014.29). Top panel:the SED is computed in the whole range of observed frequencies. Fluxdensities are obtained from Gaussian fits to 2 GHz bandwidth images withina circular region of 1.25 arcsec diameter enclosing the source. We useuniform weighting at C band and natural weighting at Ku, K, and Q bands.A fit to the measured flux densities is also shown (S ν = a ν α [1 − e − b / ν β ], witha = 0.62, α = 1.38, b = 13.29, and β = 1.42, with S ν in mJy and ν in GHz).This fitted ad hoc function is only for description purposes of the observedSED (see Section 4.1). Bottom panel: Spectrum at Q band. Flux densities areobtained from Gaussian fits to 1 GHz bandwidth images within a circularregion of 0.16 arcsec diameter. We use Briggs weighting (robust 0). Thesolid line is a linear least-squares fit to the log data, from which we derive aspectral index α = + . ± . (cid:15) = + . ± .
5, consistentwith a thermal radio jet (see Section 4.1). All data points are shown withmeasurement errors, considering both fitting and calibration uncertainties.Note that these panels trace different components in the source: while the toppanel corresponds to the HCHII region, the bottom one traces the compactjet. ported in the massive YSOs G35.20–074N (Beltrán et al. 2016) andG345.4938+01.4677 (Guzmán et al. 2016).
VLA 3 was previously proposed to be a partially optically thickcompact HII region (e.g., Torrelles et al. 1997; Shepherd, Kurtz& Testi 2004), but it was later suggested to be a thermal radio jetwith a spectral index α . − = + . ± . Figure 6.
Dependence of the angular size of the jet with frequency (toppanel) and spectral energy distribution (bottom panel) of VLA 3. Fluxdensities and semi-major axes θ ν , are obtained from Gaussian fits to thebrightness profile of VLA 3, in 2 GHz bandwidth images. To measure fluxdensities we use uniform-weighted images (C band) and natural-weightedimages (Ku, K, and Q bands), while angular sizes were measured in uniform-weighted images (C, Ku, and K bands) and Briggs-weighted (robust -1)images (Q band). Solid lines are linear least-squares fits to the log data.Angular size error bars correspond to fitting errors, while flux error barsconsider both fitting and calibration errors. Table 3.
VLA 1 PARAMETERS FROM K-BAND EMISSIONParameter Value Description ν × Hz band central frequency S ν (5.6 ± min (74 ±
3) mas = (3.6 ± × − rad source minor axisFWHM maj (100 ±
2) mas = (4.8 ± × − rad source major axisL ∼
90 mas (cid:39)
110 au source characteristic size Ω ν (2.0 ± × − sr subtended solid angleT B (1890 ±
10) K brightness temperature τ ν ∼ ∼ × cm − pc emission measure n e ∼ cm − electron number density (cid:219) N ∼ × photons s − ionizing photon rateThe flux density S , r min , r maj , and the characteristic size L correspond to values derivedfrom a by-dimensional Gaussian fit to VLA 1 at K-band (robust 0 weighting).MNRAS , 1–12 (0000) A. Rodríguez-Kamenetzky et al. 2010). In our data (Figs. 1 and 2), VLA 3 appears as anelongated source, with its major axis oriented in the northwest-southeast direction at all wavelengths, with a position angle PA =-17 ◦ ± ◦ (Fig. 1). The ionized emission is characterized by a spectralindex α (cid:39) + θ ν (top panel) and the SED (bottom panel) do vary aspower laws of the frequency. In the ideal case of a conical thermal jet,with constant velocity, temperature, and ionization fraction, valuesof +0.6 and − (cid:219) M wefollow Equation 1. In this case ν , S ν , and α correspond to the com-bined image (C+Ku+K+Q bands), being ν = 26 GHz the centralfrequency of the band, S ν = 9.06 ± α = + ν = 47 GHz, thus, we take this valueas a lower limit to the turn-over frequency ν t . As in the case ofVLA 1, we also adopt a jet inclination angle i = ◦ , and estimatelower and upper limits for the mass loss rate considering differentapproximations for the ionization fraction and the jet velocity, i.e., ∼ × − M (cid:12) yr − (assuming x o = 1 and V jet = 100 km s − ) and ∼ × − M (cid:12) yr − (assuming x o = 0.1 and V jet = 1000 km s − ),respectively. Such mass-loss rates are significantly higher than thoseestimated in low- and intermediate-mass YSOs (e.g., Beltrán et al.2001; Anglada, Rodríguez & Carrasco-González 2018), but similarto the values obtained in high-mass YSOs (e.g., Rodríguez et al.1994; Guzmán et al. 2012; Añez-López et al. 2020), supporting thatVLA 3 is excited by a massive protostar. In Fig. 1c we show the radio image with the highest resolution andsensitivity to date of Bc and VLA 4. This allows us to resolve theirstructure, and study the nature of their emission through the spectralindex map. The source Bc is clearly resolved into two components(labeled as Bc [E] and Bc [W] in Fig. 1). We note that Bc [W]–Bc [E]form an elongated structure, with its minor axis aligned with theVLA 3 jet direction as we would expect to observe in a bow-shockproduced by the impact of a supersonic jet with the environment gas(e.g., Tafalla et al. 2017; Castellanos-Ramírez, Raga & Rodríguez-González 2018). This supports the scenario proposed by Carrasco-González et al. (2010) who interpreted Bc as an obscured radio
Figure 7.
Proper motion diagrams for the Bc and VLA 4 sources. Positionsare computed as the distances to the averaged coordinates of the systemVLA 1-VLA 2-VLA 3 in four epochs (1992.98, 2001.40, 2006.47, 2014.29):RA(J2000) = 20h 38m 36.47s, DEC(J2000)= 42 ◦ (cid:48) (cid:48)(cid:48) . The solid linesare least-square fits to the data. Velocities on the plane of sky are derivedby assuming a distance to the region of 1.3 kpc (Rygl et al. 2012). Bc andVLA 4 are moving away from the system with PAs of ∼ -20 ◦ and ∼ -10 ◦ ,respectively. Herbig-Haro (HH) object, possibly excited by the VLA 3 jet. Aflattened structure similar to that of Bc is also seen in the frontalregion of the shock of the obscured HH 80N object (Rodríguez-Kamenetzky et al. 2019).Carrasco-González et al. (2010) studied the kinematics of thesesources by computing proper motions relative to VLA 3 in threeepochs (1992.90, 1998.23, and 2006.38) spanning 13.48 years.Adopting a distance to the region of 2 kpc (Dickel, Wendker &Bieritz 1969), they derived for Bc a velocity of 220 ±
70 km s − moving on the plane of the sky, and toward the south, approxi-mately along the major axis of the VLA 3 radio jet. RegardingVLA 4, they suggested it could be either an independent star orshock-excited gas produced by a previous ejection from VLA 3.However, even though they noticed a small displacement of VLA 4to the south with respect to VLA 3 between 2000 and 2006, theywere not able to distinguish between these two scenarios. Regardingthis, we measured the proper motion of both sources along a timespan of 22 years, from 1992 to 2014. Positions of the sources in 2014were measured in the K band image, since it is the highest angularresolution image where Bc and VLA 4 are detected (see Fig. 2).We compute the proper motions relative to the average position ofthe system VLA 1-VLA 2-VLA 3 assuming it is stationary, insteadof VLA 3 only. In this way, we reduce possible errors associated tovariations in the shape of VLA 3 due to observational effects (e.g.differences in the beam size at each epoch) or real changes (e.g.,new ejections from VLA 3). Although Bc shows substructure, wemeasure the displacement of the two-component complex. Fig. 7shows that both Bc and VLA 4 are moving away from the system inthe southeast direction (PA (cid:39) -20 ◦ and -10 ◦ , individually, see Fig.7) with velocities on the plane of the sky of 1.9 × − arcsec yr − ( ∼
118 km s − ), and 1.8 × − arcsec yr − ( ∼
112 km s − ), respec-tively (assuming the updated distance to the region of 1.3 kpc; Ryglet al. 2012). The velocity of Bc differs from the value given byCarrasco-González et al. (2010), but this is mostly due to the differ-ence in the adopted distance to the region (considering a distance MNRAS000
112 km s − ), respec-tively (assuming the updated distance to the region of 1.3 kpc; Ryglet al. 2012). The velocity of Bc differs from the value given byCarrasco-González et al. (2010), but this is mostly due to the differ-ence in the adopted distance to the region (considering a distance MNRAS000 , 1–12 (0000) haracterization of W75N (B) of 2 kpc we obtain velocities of 170 and 180 km s − for VLA 4and Bc, respectively, closer to the results reported by these authors).Thus, the shape and proper motions of Bc and VLA 4 are consistentwith both sources tracing shock-excited gas (obscured HH objects).Moreover, from the spectral index map (Fig. 1c) we see that Bcand VLA 4 are dominated by flat spectral indices ( α ∼ (cid:39) -17 ◦ ), consistent with the di-rection of the proper motions of Bc and VLA 4 (PA (cid:39) -20 ◦ and-10 ◦ ). This, along with the results found in section 4.2, suggeststhat VLA 3 is the driving source of Bc and VLA 4. In addition, thefact that Bc and VLA 4 are not associated with any of the detectedmillimeter cores (see Section 3.2; Fig. 3) suggests that they are notprotostars, further supporting our shock-excited gas interpretationfor these sources. This kind of obscured HH objects, exhibitingproper motions higher than 100 km s − , have also been observed inradio continuum in other intermediate- and high-mass star-formingregions: e.g., Serpens (Curiel et al. 1993; Rodríguez-Kamenetzkyet al. 2016), GGD 27 (Martí et al. 1995, 1998; Masqué et al. 2015;Rodríguez-Kamenetzky et al. 2019), Cepheus A (Curiel et al. 2006). We presented an analysis of high-sensitivity, high-resolution multi-frequency VLA observations of the massive star-forming regionW75N (B), together with complementary studies performed withALMA and VLA archive data. Our study leads us to the followingconclusions: • VLA 1 is detected at all the observed frequencies (4-48 GHz).Its SED over the entire range of frequencies is consistent withthermal free-free emission from an HCHII region ( (cid:46) (cid:46) ≈ ≈
130 au), with aspectral index α ≈ + . ν ∝ ν α ). This suggests that VLA 1 isdriving a thermal radio jet, and it is likely at the early stage of thephotoionization. • VLA 3 shows an elongated structure at scales of few tenthsof arcsec (few hundred of au), with its major axis oriented in thenorthwest–southeast direction (PA ≈ -17 ◦ ). Both the SED and thesize dependence with frequency indicates that this source is alsodriving a thermal radio jet. • We computed proper motions of the radio continuum sourcesBc and VLA 4 in a time interval of 22 years. We found both sourcesare moving away toward the south, in a similar direction as theVLA 3 thermal radio jet, with velocities of ≈ − ( ≈ × − arcsec yr − ). From the SED analysis we foundthese sources are dominated by flat spectral indices, as it is expectedfor optically thin free-free emission produced by shock-ionized ma-terial. These results support the scenario in which Bc and VLA 4are obscured HH objects tracing shocks of the jet driven by VLA 3. • Four 1.3 mm continuum cores are observed with ALMA(MM1, MM2, MM3, and MM[N]) in a region of ∼
14 arcsec. Threeof these millimeter cores, MM1, MM2, and MM3, had previously been identified with the SMA interferometer, while MM[N] had notbeen previously reported. VLA 1, VLA 2, and VLA 3 are associatedwith the brightest core MM1. Bc and VLA 4 are not associated withany of the millimeter continuum cores, supporting they are not YSOsbut shock-excited gas as concluded from our VLA observations. • We have detected three new weak compact radio continuumsources (VLA[SW], VLA[NE], and Bd). Two of them, VLA[SW]and VLA[NE] are associated with the millimeter cores MM2 andMM3, respectively, suggesting they are embedded YSOs belongingto the W75N (B) massive star-forming region. • With our VLA observations we have identified a cluster of atleast five YSOs (VLA 1, VLA 2, VLA 3, VLA[SW] and VLA[NE])in a region of ∼
10 arsec ( ∼ Data availability
The datasets underlying this article were derived fromsources in the public domain: NRAO Data Archive,https://science.nrao.edu/observing/data-archive.
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APPENDIX A: SUPPLEMENTARY TABLES ANDFIGURES
Table A1 lists the parameters of the images used in this paper. Thecolumns are as follows: [1] spectral band, [2] epoch of observation,[3] central frequency, [4] bandwidth, [5] weighting of visibilities(NA = natural, UN = uniform, and R0 and R-1 = Briggs, using robust parameter equal to 0 and -1), [6] synthesized beam size andposition angle, [7] rms noise, [8] Figure/Table in which each imageis used in the paper.Fig. A1 shows a radio continuum map of the region, with close-ups of the three new compact sources of < µ Jy detected in thefield: VLA[NE], VLA[SW], and Bd.
This paper has been typeset from a TEX/L A TEX file prepared by the author.MNRAS000
This paper has been typeset from a TEX/L A TEX file prepared by the author.MNRAS000 , 1–12 (0000) haracterization of W75N (B) Table A1.
PARAMETERS OF THE VLA IMAGESSpectral Band Epoch Central Frequency Total Bandwidth Weighting Synthesized rms Used in(GHz) (GHz) Beam ( µ Jy/beam)X 1992.90 8.44 0.1 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
19; -56 ◦
90 Fig.7Ku 2001.31 15.0 0.1 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
39; -83 ◦
80 Fig.7X 2006.38 8.46 0.1 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
17; 18 ◦
50 Fig.7C 2014.29 6.0 4.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
20; 83 ◦
30 Fig.2, Table 2Ku 2014.29 15.0 6.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
16; -75 ◦
10 Fig.2, 4, Table 2K 2014.29 22.0 8.5 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
10; -74 ◦
10 Fig.2, 4, 7, Table 2K 2014.29 22.0 8.5 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
07; -77 ◦
10 Table 3Q 2014.29 44.0 10.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
05; -29 ◦
20 Fig.2, 4, Table 2Q 2014.29 44.0 10.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
05; -9 ◦
100 Fig.4C+Ku+K+Q 2014.29 25.9 28.5 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09; -69 ◦ . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
90 Fig.5C 2014.29 7.0 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
90 Fig.5C 2014.29 7.0 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
20; 87 ◦
50 Fig.6Ku 2014.29 13.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
90 Fig.5Ku 2014.29 13.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
19; -85 ◦
45 Fig.6Ku 2014.29 13.0 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
11; 90 ◦
120 Fig.6Ku 2014.29 15.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
100 Fig.5Ku 2014.29 15.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
15; -77 ◦
35 Fig.6Ku 2014.29 15.0 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09; 89 ◦
100 Fig.6Ku 2014.29 17.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
100 Fig.5Ku 2014.29 17.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
14; -66 ◦
30 Fig.6Ku 2014.29 17.0 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09; -87 ◦
90 Fig.6K 2014.29 19.4 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
90 Fig.5K 2014.29 19.4 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
11; -74 ◦
20 Fig.6K 2014.29 19.4 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
07; -84 ◦
75 Fig.6K 2014.29 21.2 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
100 Fig.5K 2014.29 21.2 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
10; -75 ◦
20 Fig. 6K 2014.29 21.2 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
160 Fig.6K 2014.29 22.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
150 Fig.5K 2014.29 22.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
09; -73 ◦
30 Fig.6K 2014.29 22.0 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
130 Fig.6K 2014.29 23.4 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
120 Fig.5K 2014.29 23.4 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
10; -74 ◦
20 Fig.6K 2014.29 23.4 2.0 UN 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
90 Fig.6Q 2014.29 41.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
220 Fig.5Q 2014.29 41.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
30 Fig.6Q 2014.29 41.0 2.0 R-1 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
70 Fig.6Q 2014.29 43.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
200 Fig.5Q 2014.29 43.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
05; -27 ◦
30 Fig.6Q 2014.29 43.0 2.0 R-1 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
80 Fig.6Q 2014.29 45.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
300 Fig.5Q 2014.29 45.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
05; -25 ◦
40 Fig.6Q 2014.29 45.0 2.0 R-1 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
100 Fig.6Q 2014.29 47.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
37; 0 ◦
350 Fig.5Q 2014.29 47.0 2.0 NA 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
05; -18 ◦
70 Fig.6Q 2014.29 47.0 2.0 R-1 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
200 Fig.6Q 2014.29 40.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
50 Fig.5Q 2014.29 41.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
50 Fig.5Q 2014.29 42.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
55 Fig.5Q 2014.29 43.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
65 Fig.5Q 2014.29 44.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
65 Fig.5Q 2014.29 45.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
80 Fig.5Q 2014.29 46.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
100 Fig.5Q 2014.29 47.5 1.0 R0 0 . (cid:48)(cid:48) × . (cid:48)(cid:48) ◦
180 Fig.5MNRAS , 1–12 (0000) A. Rodríguez-Kamenetzky
Figure A1.
Radio continuum image of W75N (B) made by the combination of C, Ku, K, and Q bands (epoch 2014.29), using multifrequency synthesiscleaning and Briggs weighting (robust 0). Three new weak ( < µ Jy) compact radio continuum sources are detected: VLA[NE] and VLA[SW] located at ∼ ∼ ∼ × − Jy/beam (VLA[NE]); 4, 7, 10, and 13 times 7 × − Jy/beam (VLA[SW]); and 4, 7, and 9 times 7 × − Jy/beam (Bd). Black rectanglesshow zoomed-in regions enclosing each of the new sources, where the physical scale is given by the syntesized beam, corresponding to 160 au ×
120 au (PA =–69 ◦ ), approximately. MNRAS000