arXiv subjects

M. G. Hoare

Publications and source records attributed to M. G. Hoare.

At least 37 records · Page 2Linked to original sources

arXiv2017

Medium resolution near-infrared spectroscopy of Massive Young Stellar Objects

We present medium-resolution (R~7000) near-infrared echelle spectroscopic data for 36 MYSOs drawn from the Red MSX Source (RMS) survey. This is the largest sample observed at this resolution at these wavelengths of MYSOs to date. The spectra are characterized mostly by emission from hydrogen recombination lines and accretion diagnostic lines. One MYSO shows photospheric HI absorption, a comparison with spectral standards indicates that the star is an A type star with a low surface gravity, implying that the MYSOs are probably swollen, as also suggested by evolutionary calculations. An investigation of the Br$γ$ line profiles shows that most are in pure emission, while 13$\pm$5 % display P Cygni profiles, indicative of outflow, while less than 8$\pm$4 % have inverse P Cygni profiles, indicative of infall. These values are comparable with investigations into the optically bright Herbig Be stars, but not with those of Herbig Ae and T Tauri stars, consistent with the notion that the more massive stars undergo accretion in a different fashion than lower mass objects which are undergoing magnetospheric accretion. Accretion luminosities and rates as derived from the Br$γ$ line luminosities agree with results for lower mass sources, providing tentative evidence for massive star formation theories based on scaling of low-mass scenarios. We present Br$γ$/Br12 line profile ratios exploiting the fact that optical depth effects can be traced as a function of Doppler shift across the lines. These show that the winds of MYSOs in this sample are nearly equally split between constant, accelerating, and decelerating velocity structures. There are no trends between the types of features we see and bolometric luminosities or near-infrared colours.

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arXiv2017

Properties of Hi-GAL clumps in the inner Galaxy]{The Hi-GAL compact source catalogue. I. The physical properties of the clumps in the inner Galaxy ($-71.0^{\circ}< \ell < 67.0^{\circ}$)

Hi-GAL is a large-scale survey of the Galactic plane, performed with Herschel in five infrared continuum bands between 70 and 500 $μ$m. We present a band-merged catalogue of spatially matched sources and their properties derived from fits to the spectral energy distributions (SEDs) and heliocentric distances, based on the photometric catalogs presented in Molinari et al. (2016a), covering the portion of Galactic plane $-71.0^{\circ}< \ell < 67.0^{\circ}$. The band-merged catalogue contains 100922 sources with a regular SED, 24584 of which show a 70 $μ$m counterpart and are thus considered proto-stellar, while the remainder are considered starless. Thanks to this huge number of sources, we are able to carry out a preliminary analysis of early stages of star formation, identifying the conditions that characterise different evolutionary phases on a statistically significant basis. We calculate surface densities to investigate the gravitational stability of clumps and their potential to form massive stars. We also explore evolutionary status metrics such as the dust temperature, luminosity and bolometric temperature, finding that these are higher in proto-stellar sources compared to pre-stellar ones. The surface density of sources follows an increasing trend as they evolve from pre-stellar to proto-stellar, but then it is found to decrease again in the majority of the most evolved clumps. Finally, we study the physical parameters of sources with respect to Galactic longitude and the association with spiral arms, finding only minor or no differences between the average evolutionary status of sources in the fourth and first Galactic quadrants, or between "on-arm" and "inter-arm" positions.

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arXiv2017

H2O Southern Galactic Plane Survey (HOPS): Paper III - Properties of Dense Molecular Gas across the Inner Milky Way

The H2O Southern Galactic Plane Survey (HOPS) has mapped 100 square degrees of the Galactic plane for water masers and thermal molecular line emission using the 22-m Mopra telescope. We describe the automated spectral-line fitting pipelines used to determine the properties of emission detected in HOPS datacubes, and use these to derive the physical and kinematic properties of gas in the survey. A combination of the angular resolution, sensitivity, velocity resolution and high critical density of lines targeted make the HOPS data cubes ideally suited to finding precursor clouds to the most massive and dense stellar clusters in the Galaxy. We compile a list of the most massive HOPS ammonia regions and investigate whether any may be young massive cluster progenitor gas clouds. HOPS is also ideally suited to trace the flows of dense gas in the Galactic Centre. We find the kinematic structure of gas within the inner 500pc of the Galaxy is consistent with recent predictions for the dynamical evolution of gas flows in the centre of the Milky Way. We confirm a recent finding that the dense gas in the inner 100pc has an oscillatory kinematic structure with characteristic length scale of ~20pc, and also identify similar oscillatory kinematic structure in the gas at radii larger than 100pc. Finally, we make all of the above fits and the remaining HOPS data cubes across the 100 square degrees of the survey available to the community.

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arXiv2017

The JCMT Plane Survey: First complete data release - emission maps and compact source catalogue

We present the first data release of the James Clerk Maxwell Telescope (JCMT) Plane Survey (JPS), the JPS Public Release 1 (JPSPR1). JPS is an 850-um continuum survey of six fields in the northern inner Galactic Plane in a longitude range of l=7-63, made with the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2). This first data release consists of emission maps of the six JPS regions with an average pixel-to-pixel noise of 7.19 mJy beam^-1, when smoothed over the beam, and a compact-source catalogue containing 7,813 sources. The 95 per cent completeness limits of the catalogue are estimated at 0.04 Jy beam^-1 and 0.3 Jy for the peak and integrated flux densities, respectively. The emission contained in the compact-source catalogue is 42 +- 5 per cent of the total and, apart from the large-scale (greater than 8') emission, there is excellent correspondence with features in the 500-um Herschel maps. We find that, with two-dimensional matching, 98 +- 2 per cent of sources within the fields centred at l=20, 30, 40 and 50 are associated with molecular clouds, with 91 +- 3 per cent of the l=30 and 40 sources associated with dense molecular clumps. Matching the JPS catalogue to Herschel 70-um sources, we find that 38 +- 1 per cent of sources show evidence of ongoing star formation. The images and catalogue will be a valuable resource for studies of star formation in the Galaxy and the role of environment and spiral arms in the star formation process.

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arXiv2017

The ALMA view of W33A: A Spiral Filament Feeding the Candidate Disc in MM1-Main

We targeted the massive star forming region W33A using the Atacama Large Sub/Millimeter Array (ALMA) in band 6 (230 GHz) and 7 (345 GHz) to search for a sub-1000au disc around the central O-type massive young stellar object (MYSO) W33A MM1-Main. Our data achieves a resolution of ~0.2" (~500au) and resolves the central core, MM1, into multiple components and reveals complex and filamentary structures. There is strong molecular line emission covering the entire MM1 region. The kinematic signatures are inconsistent with only Keplerian rotation although we propose that the shift in the emission line centroids within ~1000au of MM1-Main could hint at an underlying compact disc with Keplerian rotation. We cannot however rule out the possibility of an unresolved binary or multiple system. A putative smaller disc could be fed by the large scale spiral `feeding filament' we detect in both gas and dust emission. We also discuss the nature of the now-resolved continuum sources.

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arXiv2016

Connecting low- and high-mass star formation: the intermediate-mass protostar IRAS 05373+2349 VLA 2

Until recently, there have been few studies of the protostellar evolution of intermediate-mass (IM) stars, which may bridge the low- and high-mass regimes. This paper aims to investigate whether the properties of an IM protostar within the IRAS 05373+2349 embedded cluster are similar to that of low- and/or high-mass protostars. We carried out Very Large Array as well as Combined Array for Research in Millimeter Astronomy continuum and 12CO(J=1-0) observations, which uncover seven radio continuum sources (VLA 1-7). The spectral index of VLA 2, associated with the IM protostar is consistent with an ionised stellar wind or jet. The source VLA 3 is coincident with previously observed H2 emission line objects aligned in the north-south direction (P.A. -20 to -12 deg), which may be either an ionised jet emanating from VLA 2 or (shock-)ionised cavity walls in the large-scale outflow from VLA 2. The position angle between VLA 2 and 3 is slightly misaligned with the large-scale outflow we map at ~5-arcsec resolution in 12CO (P.A. ~30 deg), which in the case of a jet suggests precession. The emission from the mm core associated with VLA 2 is also detected; we estimate its mass to be 12-23 Msun, depending on the contribution from ionised gas. Furthermore, the large-scale outflow has properties intermediate between outflows from low- and high-mass young stars. Therefore, we conclude that the IM protostar within IRAS 05373+2349 is phenomenologically as well as quantitatively intermediate between the low- and high-mass domains.

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arXiv2016

The prevalence of star formation as a function of Galactocentric radius

We present large-scale trends in the distribution of star-forming objects revealed by the Hi-GAL survey. As a simple metric probing the prevalence of star formation in Hi-GAL sources, we define the fraction of the total number of Hi-GAL sources with a 70-micron counterpart as the "star-forming fraction" or SFF. The mean SFF in the inner galactic disc (3.1 kpc < R_GC < 8.6 kpc) is 25%. Despite an apparent pile-up of source numbers at radii associated with spiral arms, the SFF shows no significant deviations at these radii, indicating that the arms do not affect the star-forming productivity of dense clumps either via physical triggering processes or through the statistical effects of larger source samples associated with the arms. Within this range of Galactocentric radii, we find that the SFF declines with R_GC at a rate of -0.026 +/- 0.002 per kiloparsec, despite the dense gas mass fraction having been observed to be constant in the inner Galaxy. This suggests that the SFF may be weakly dependent on one or more large-scale physical properties of the Galaxy, such as metallicity, radiation field, pressure or shear, such that the dense sub-structures of molecular clouds acquire some internal properties inherited from their environment.

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arXiv2016

A search for ionized jets towards massive young stellar objects

Radio continuum observations using the Australia telescope compact array at 5.5, 9.0, 17.0 and 22.8 GHz have detected free-free emission associated with 45 of 49 massive young stellar objects and HII regions. Of these, 26 sources are classified as ionized jets (12 of which are candidates), 2 as ambiguous jets or disc winds, 1 as a disc-wind, 14 as HII regions and 2 were unable to be categorised. Classification as ionized jets is based upon morphology, radio flux and spectral index, in conjunction with previous observational results at other wavelengths. Radio-luminosity and momentum are found to scale with bolometric luminosity in the same way as low-mass jets, indicating a common mechanism for jet production across all masses. In 13 of the jets, we see associated non-thermal/optically-thin lobes resulting from shocks either internal to the jet and/or at working surfaces. Ten jets display non-thermal (synchrotron emission) spectra in their lobes, with an average spectral index of -0.55 consistent with Fermi acceleration in shocks. This shows that magnetic fields are present, in agreement with models of jet formation incorporating magnetic fields. Since the production of collimated radio jets is associated with accretion processes, the results presented in this paper support the picture of disc-mediated accretion for the formation of massive stars with an upper-limit on the jet phase lasting approximately $6.5 \times 10^4 yr$. Typical mass loss rates in the jet are found to be $1.4 \times 10^{-5} M_\odot yr^{-1}$ with associated momentum rates of the order $(1-2) \times 10^{-2} M_\odot km s^{-1} yr^{-1}$.

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arXiv2016

Hi-GAL, the Herschel infrared Galactic Plane Survey: photometric maps and compact source catalogues. First data release for Inner Milky Way: +68°> l > -70°

(Abridged) We present the first public release of high-quality data products (DR1) from Hi-GAL, the {\em Herschel} infrared Galactic Plane Survey. Hi-GAL is the keystone of a suite of continuum Galactic Plane surveys from the near-IR to the radio, and covers five wavebands at 70, 160, 250, 350 and 500 micron, encompassing the peak of the spectral energy distribution of cold dust for 8 < T < 50K. This first Hi-GAL data release covers the inner Milky Way in the longitude range 68° > l > -70° in a |b|<1° latitude strip. Photometric maps have been produced with the ROMAGAL pipeline, that optimally capitalizes on the excellent sensitivity and stability of the bolometer arrays of the {\em Herschel} PACS and SPIRE photometric cameras, to deliver images of exquisite quality and dynamical range, absolutely calibrated with {\em Planck} and {\em IRAS}, and recovering extended emission at all wavelengths and all spatial scales. The compact source catalogues have been generated with the CuTEx algorithm, specifically developed to optimize source detection and extraction in the extreme conditions of intense and spatially varying background that are found in the Galactic Plane in the thermal infrared. Hi-GAL DR1 images will be accessible via a dedicated web-based image cutout service. The DR1 Compact Source Catalogues are delivered as single-band photometric lists containing, in addition to source position, peak and integrated flux and source sizes, a variety of parameters useful to assess the quality and reliability of the extracted sources, caveats and hints to help this assessment are provided. Flux completeness limits in all bands are determined from extensive synthetic source experiments and depend on the specific line of sight along the Galactic Plane. Hi-GAL DR1 catalogues contain 123210, 308509, 280685, 160972 and 85460 compact sources in the five bands, respectively.

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arXiv2015

A distance limited sample of massive molecular outflows

We have observed 99 mid-infrared-bright, massive young stellar objects and compact HII regions drawn from the Red MSX source (RMS) survey in the J=3$-$2 transition of $^{12}$CO and $^{13}$CO, using the James Clerk Maxwell Telescope. 89 targets are within 6 kpc of the Sun, covering a representative range of luminosities and core masses. These constitute a relatively unbiased sample of bipolar molecular outflows associated with massive star formation. Of these, 59, 17 and 13 sources (66, 19 and 15 percent) are found to have outflows, show some evidence of outflow, and have no evidence of outflow, respectively. The time-dependent parameters of the high-velocity molecular flows are calculated using a spatially variable dynamic timescale. The canonical correlations between the outflow parameters and source luminosity are recovered and shown to scale with those of low-mass sources. For coeval star formation we find the scaling is consistent with all the protostars in an embedded cluster providing the outflow force, with massive stars up to $\sim$30 M$_{\odot}$ generating outflows. Taken at face value, the results support the model of a scaled-up version of the accretion-related outflow-generation mechanism associated with discs and jets in low-mass objects with time-averaged accretion rates of $\sim$10$^{-3}$ M$_{\odot}$ yr$^{-1}$ onto the cores. However, we also suggest an alternative model, in which the molecular outflow dynamics are dominated by the entrained mass and are unrelated to the details of the acceleration mechanism. We find no evidence that outflows contribute significantly to the turbulent kinetic energy of the surrounding dense cores.

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arXiv2015

The JCMT Plane Survey: early results from the l = 30 degree field

We present early results from the JCMT Plane Survey (JPS), which has surveyed the northern inner Galactic plane between longitudes l=7 and l=63 degrees in the 850-μm continuum with SCUBA-2, as part of the James Clerk Maxwell Telescope Legacy Survey programme. Data from the l=30 degree survey region, which contains the massive star-forming regions W43 and G29.96, are analysed after approximately 40% of the observations had been completed. The pixel-to-pixel noise is found to be 19 mJy/beam, after a smooth over the beam area, and the projected equivalent noise levels in the final survey are expected to be around 10 mJy/beam. An initial extraction of compact sources was performed using the FellWalker method resulting in the detection of 1029 sources above a 5-σ surface-brightness threshold. The completeness limits in these data are estimated to be around 0.2 Jy/beam (peak flux density) and 0.8 Jy (integrated flux density) and are therefore probably already dominated by source confusion in this relatively crowded section of the survey. The flux densities of extracted compact sources are consistent with those of matching detections in the shallower ATLASGAL survey. We analyse the virial and evolutionary state of the detected clumps in the W43 star-forming complex and find that they appear younger than the Galactic-plane average.

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arXiv2015

Herschel Hi-GAL imaging of massive young stellar objects

We used Herschel Hi-GAL survey data to determine whether massive young stellar objects (MYSOs) are resolved at 70$μ$m and to study their envelope density distribution. Our analysis of three relatively isolated sources in the l=30° and l=59° Galactic fields show that the objects are partially resolved at 70$μ$m. The Herschel Hi-GAL survey data have a high scan velocity which makes unresolved and partially resolved sources appear elongated in the 70$μ$m images. We analysed the two scan directions separately and examine the intensity profile perpendicular to the scan direction. Spherically symmetric radiative transfer models with a power law density distribution were used to study the circumstellar matter distribution. Single dish sub-mm data were also included to study how different spatial information affects the fitted density distribution. The density distribution which best fits both the 70$μ$m intensity profile and SED has an average index of ~0.5. This index is shallower than expected and is probably due to the dust emission from bipolar outflow cavity walls not accounted for in the spherical models. We conclude that 2D axisymmetric models and Herschel images at low scan speeds are needed to better constrain the matter distribution around MYSOs.

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arXiv2014

ATLASGAL --- towards a complete sample of massive star forming clumps

By matching infrared-selected, massive young stellar objects (MYSOs) and compact HII regions in the RMS survey to massive clumps found in the submillimetre ATLASGAL survey, we have identified ~1000 embedded young massive stars between 280\degr < $\ell$ < 350\degr and 10degr < $\ell$ < 60\degr with |b|<1.5degr. Combined with an existing sample of radio-selected methanol masers and compact HII regions, the result is a catalogue of ~1700 massive stars embedded within ~1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and HII-region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no significant differences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of star formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and HII-region subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs associated with the most massive clumps, probably due to very short pre-main sequence lifetimes in the most luminous sources.

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arXiv2013

The RMS Survey: Galactic distribution of massive star formation

Abridged: We have used the well-selected sample of ~1750 embedded, young, massive stars identified by the RMS survey to investigate the Galactic distribution of recent massive star formation. We describe the various methods used to assign distances extracted from the literature, and solve the distance ambiguities towards ~200 sources located within the Solar circle using archival HI data. These distances are used to calculate bolometric luminosities and estimate the survey completeness (~2x10^4 lsun). In total, we calculate the distance and luminosity of ~1650 sources, one third of which are above the survey's completeness threshold. Examination of the sample's longitude, latitude, radial velocities and mid-infrared images has identified ~120 small groups of sources, many of which are associated with well known star formation complexes, such as W43, W49 and W51. We compare the positional distribution of the sample with the expected locations of the spiral arms, assuming a model of the Galaxy consisting of four gaseous arms. The distribution of young massive stars in the Milky Way is spatially correlated with the spiral arms, with strong peaks in the source position and luminosity distributions at the arms' Galactocentric radii. The overall source and luminosity surface densities are both well correlated with the surface density of the molecular gas, which suggests that the massive star formation rate per unit molecular mass is approximately constant across the Galaxy. We estimate the total luminosity of the embedded massive star population to be ~0.76x10^8 lsun, 30% of which is associated with the ten most active star forming complexes. We measure the scale height as a function of Galactocentric distance and find that it increases only modestly from ~20-30 pc between 4 and 8 kpc, but much more rapidly at larger distances.

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arXiv2013

Near-infrared integral field spectroscopy of Massive Young Stellar Objects

We present medium resolution ($R\approx5300$) $K$-band integral field spectroscopy of six MYSOs. The targets are selected from the RMS survey, and we used the NIFS on the Gemini North telescope. The data show various spectral line features including Br$γ$, CO, H$_2$, and \mbox{He\,{\sc i}}. The Br$γ$ line is detected in emission in all objects with $v_\mathrm{FWHM}\sim100$ -- 200 kms$^{-1}$. V645 Cyg shows a high-velocity P-Cygni profile between -800 kms$^{-1}$ and -300 kms$^{-1}$. We performed three-dimensional spectroastrometry to diagnose the circumstellar environment in the vicinity of the central stars using the Br$γ$ line. We measured the centroids of the velocity components with sub-mas precision. The centroids allow us to discriminate the blueshifted and redshifted components in a roughly east--west direction in both IRAS 18151--1208 and S106 in Br$γ$. This lies almost perpendicular to observed larger scale outflows. We conclude, given the widths of the lines and the orientation of the spectroastrometric signature, that our results trace a disc wind in both IRAS 18151--1208 and S106. The CO $ν=2-0$ absorption lines at low $J$ transitions are detected in IRAS 18151--1208 and AFGL 2136. We analysed the velocity structure of the neutral gas discs. In IRAS 18151--1208, the absorption centroids of the blueshifted and redshifted components are separated in a direction of north-east to south-west, nearly perpendicular to that of the larger scale $H_2$ jet. The position-velocity relations of these objects can be reproduced with central masses of 30 $M_{\sun}$ for IRAS 18151--1208 and 20 $M_{\sun}$ for AFGL 2136. We also detect CO $ν=2-0$ bandhead emission in IRAS 18151--1208, S106 and V645 Cyg. The results can be fitted reasonably with a Keplerian rotation model, with masses of 15, 20 and 20 $M_{\sun}$ respectively.

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arXiv2013

The Red MSX Source Survey: the Massive Young Stellar Population of our Galaxy

We present the Red MSX Source (RMS) Survey, the largest statistically selected catalog of young massive protostars and HII regions to date. We outline the construction of the catalog using mid and near infrared color selection, as well as the detailed follow up work at other wavelengths, and at higher spatial resolution in the infrared. We show that within the adopted selection bounds we are more than 90% complete for the massive protostellar population, with a positional accuracy of the exciting source of better than 2 arcseconds. We briefly summarize some of the results that can be obtained from studying the properties of the objects in the catalog as a whole, and find evidence that the most massive stars form: (i) preferentially nearer the Galactic centre than the anti-centre; (ii) in the most heavily reddened environments, suggestive of high accretion rates; and (iii) from the most massive cloud cores.

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arXiv2013

ATLASGAL --- properties of compact HII regions and their natal clumps

We present a complete sample of molecular clumps containing compact and ultra-compact (UC) HII regions between \ell=10\degr and 60\degr\ and $|b|<1\degr, identified by combining the the ATLASGAL submm and CORNISH radio continuum surveys with visual examination of archival infrared data. Our sample is complete to optically thin, compact and UCHII regions driven by a zero age main sequence star of spectral type B0 or earlier embedded within a 1,000 Msun clump. In total we identify 213 compact and UCHII regions, associated with 170 clumps. Unambiguous kinematic distances are derived for these clumps and used to estimate their masses and physical sizes, as well as the Lyman continuum fluxes and sizes of their embedded HII regions. We find a clear lower envelope for the surface density of molecular clumps hosting massive star formation of 0.05 g cm^{-2}, which is consistent with a similar sample of clumps associated with 6.7 GHz masers. The mass of the most massive embedded stars is closely correlated with the mass of their natal clump. Young B stars appear to be significantly more luminous in the ultraviolet than predicted by current stellar atmosphere models. The properties of clumps associated with compact and UCHII regions are very similar to those associated with 6.7 GHz methanol masers and we speculate that there is little evolution in the structure of the molecular clumps between these two phases. Finally, we identify a significant peak in the surface density of compact and UCHII regions associated with the W49A star-forming complex, noting that this complex is truly one of the most massive and intense regions of star formation in the Galaxy.

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arXiv2013

The RMS Survey: Near-IR Spectroscopy of Massive Young Stellar Objects

Near-infrared H- and K-band spectra are presented for 247 objects, selected from the Red MSX Source (RMS) survey as potential young stellar objects (YSOs). 195 (~80%) of the targets are YSOs, of which 131 are massive YSOs (L_BOL > 5x10^3 L_solar), M > 8M_solar. This is the largest spectroscopic study of massive YSOs to date, providing a valuable resource for the study of massive star formation. In this paper we present our exploratory analysis of the data. The YSOs observed have a wide range of embeddedness (2.7 < A_V < 114), demonstrating that this study covers minimally obscured objects right through to very red, dusty sources. Almost all YSOs show some evidence for emission lines, though there is a wide variety of observed properties. The most commonly detected lines are Brgamma, H_2, fluorescent FeII, CO bandhead, [FeII] and HeI 2-1 2^1S-2^1P, in order of frequency of occurrence. In total, ~40% of the YSOs display either fluorescent FeII 1.6878um or CO bandhead emission (or both), indicative of a circumstellar disc; however, no correlation of the strength of these lines with bolometric luminosity was found. We also find that ~60% of the sources exhibit [FeII] or H_2 emission, indicating the presence of an outflow. Three quarters of all sources have Brgamma in emission. A good correlation with bolometric luminosity was observed for both the Brgamma and H_2 emission line strengths, covering 1 L_solar< L_BOL < 3.5x10^5 L_solar. This suggests that the emission mechanism for these lines is the same for low-, intermediate-, and high-mass YSOs, i.e. high-mass YSOs appear to resemble scaled-up versions of low-mass YSOs.

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