High-mass Protostellar Candidates Research Articles

Evidence for a cloud–cloud collision in Sh2-233 triggering the formation of the high-mass protostar object IRAS 05358+3543

ABSTRACT We have carried out a new kinematical analysis of the molecular gas in the Sh2-233 region by using the CO J = 2–1 data taken at ∼0.5 pc resolution. The molecular gas consists of a filamentary cloud of 5-pc length with 1.5-pc width where two dense cloud cores are embedded. The filament lies between two clouds, which have a velocity difference of 2.7 km s−1 and are extended over ∼5 pc. We frame a scenario that the two clouds are colliding with each other and compressed the gas between them to form the filament in ∼0.5 Myr which is perpendicular to the collision. It is likely that the collision formed not only the filamentary cloud but also the two dense cores. One of the dense cores is associated with the high-mass protostellar candidate IRAS 05358+3543, a representative high-mass protostar. In the monolithic collapse scheme of high mass star formation, a compact dense core of 100 M⊙ within a volume of 0.1 pc radius is assumed as the initial condition, whereas the formation of such a core remained unexplained in the previous works. We argue that the proposed collision is a step which efficiently collects the gas of 100 M⊙ into 0.1 pc radius. This lends support for that the cloud–cloud collision is an essential process in forming the compact high-mass dense core, IRAS 05358+3543.

Discovery of a Highly Collimated Flow from the High-mass Protostar ISOSS J23053+5953 SMM2

Abstract We present Very Large Array C-, X-, and Q-band continuum observations, as well as 1.3 mm continuum and CO(2-1) observations with the Submillimeter Array toward the high-mass protostellar candidate ISOSS J23053+5953 SMM2. Compact centimeter continuum emission was detected near the center of the SMM2 core with a spectral index of 0.24(± 0.15) between 6 and 3.6 cm, and a radio luminosity of 1.3(±0.4) mJy kpc2. The 1.3 mm thermal dust emission indicates a mass of the SMM2 core of 45.8 (±13.4) M ⊙, and a density of 7.1 (±1.2)× 106 cm−3. The CO(2-1) observations reveal a large, massive molecular outflow centered on the SMM2 core. This fast outflow (>50 km s−1 from the cloud systemic velocity) is highly collimated, with a broader, lower-velocity component. The large values for outflow mass (45.2 ± 12.6 M ⊙) and momentum rate (6 ± 2 × 10−3 M ⊙ km s−1yr−1) derived from the CO emission are consistent with those of flows driven by high-mass YSOs. The dynamical timescale of the flow is between 1.5 and 7.2 × 104 yr. We also found from the C18O to thermal dust emission ratio that CO is depleted by a factor of about 20, possibly due to freeze-out of CO molecules on dust grains. Our data are consistent with previous findings that ISOSS J23053 + 5953 SMM2 is an emerging high-mass protostar in an early phase of evolution, with an ionized jet and a fast, highly collimated, and massive outflow.

A Highly Collimated Flow from the High-mass Protostar ISOSS J23053+5953 SMM2

Abstract We present Very Large Array cm continuum observations, as well as 1.3 mm continuum and CO(2–1) observations taken with the Submillimeter Array toward the high-mass protostellar candidate ISOSS J23053+5953 SMM2. Compact cm continuum emission was detected near the center of the SMM2 dust core, and the 1.3 mm thermal dust emission indicates a core mass of 46 M ⊙. The CO(2–1) observation revealed a large, massive molecular flow centered on the SMM2 core. This fast outflow (>50 km s−1) appears highly collimated, with a broader, lower-velocity component. The large values for outflow mass (45 M ⊙), and energetics derived are consistent with those of flows driven by high-mass YSOs. The dynamical timescale of the flow is between 1.5−7.2 × 104 yr. Our data confirm previous findings that SMM2 is an emerging high-mass protostar in a very early phase of evolution, with an ionized jet, and a fast, highly collimated, and massive outflow.

MHOs toward HMOs: A Search for Molecular Hydrogen Emission-Line Objects toward High-mass Outflows

Abstract We present the results of a narrow-band near-infrared imaging survey for Molecular Hydrogen emission-line Objects (MHOs) toward 26 regions containing high-mass protostellar candidates and massive molecular outflows. We have detected a total of 236 MHOs, 156 of which are new detections, in 22 out of the 26 regions. We use H2 2.12 μm/H2 2.25 μm flux ratios, together with morphology, to separate the signatures of fluorescence associated with photo-dissociation regions (PDRs) from shocks associated with outflows in order to identify the MHOs. PDRs have typical low flux ratios of ∼1.5–3, while the vast majority of MHOs display flux ratios typical of C-type shocks (∼6–20). A few MHOs exhibit flux ratios consistent with expected values for J-type shocks (∼3–4), but these are located in regions that may be contaminated with fluorescent emission. Some previously reported MHOs have low flux ratios, and are likely parts of PDRs rather than shocks indicative of outflows. We identify a total of 36 outflows across the 22 target regions where MHOs were detected. In over half these regions, MHO arrangements and fluorescent structures trace features present in CO outflow maps, suggesting that the CO emission traces a combination of dynamical effects, which may include gas entrained in expanding PDRs as well as bipolar outflows. Where possible, we link MHO complexes to distinct outflows and identify candidate driving sources.

12CO J=2−1 and 12CO J=3−2 observations toward the high-mass protostellar candidate IRAS 20188+3928

We have carried out observations of 12CO J=2−1 and 12CO J=3−2 toward the high-mass protostellar candidate IRAS 20188+3928. Compared with previous observations, the 12CO J=2−1 and 12CO J=3−2 lines both have asymmetric profiles with an absorption dip. The velocity of the absorption dip is ∼ 1.0 km s−1. The spectral shape may be caused by rotation. The velocity-integrated intensity map and position-velocity diagram of the 12CO J=2−1 line present an obvious bipolar component, further verifying that this region has an outflow motion. This region is also associated with an HII region, an IRAS source, and an H2O maser. The H2O maser has the velocity of 1.1 km s−1. Compared with the components of the outflow, we find that the H2O maser is not associated with the outflow. Using the large velocity gradient model, we concluded that possible averaged gas densities of the blueshifted lobe and redshifted lobe are 1.0×105 cm−3 and 2.0×104 cm−3, while kinetic temperatures are 26.9 K and 52.9 K, respectively. Additionally, the outflow has a higher integrated intensity ratio (ICO J=3−2/ICO J=2−1).

Deuterium Fractionation and Ionization Degree in Massive Protostellar/cluster Cores

AbstractWe have conducted a survey of deuterium fractionation of N2H+, RD(N2H+) ≡ N(N2D+)/N(N2H+), with the Arizona Radio Observatory (ARO) Submillimeter Telescope (SMT) to assess the use of RD(N2H+) as an evolutionary tracer among massive protostellar/cluster cores in early stages. Our sample includes 32 dense cores in various evolutionary stages, from high-mass starless cores (HMSCs), high-mass protostellar objects (HMPOs), to ultra-compact (UC) HII regions, in infrared dark clouds (IRDCs) and high infrared extinction clouds. The results show a decreasing trend in deuterium fractionation with evolutionary stage traced by gas temperature and line width (Fig. 1). A moderate increasing trend of deuterium fractionation with the CO depletion factor is also found among cores in IRDCs and HMSCs. These suggest a general chemical behavior of deuterated species in low- and high-mass protostellar candidates. Upper limits to the ionization degree are also estimated to be in the range of 4 × 10−8 − 5 × 10−6.

Temperature and kinematics of protoclusters with intermediate and high-mass stars: the case of IRAS 05345+3157

We have mapped at small spatial scales the temperature and the velocity field in the protocluster associated with IRAS 05345+3157, which contains both intermediate-/high-mass protostellar candidates and starless condensations, and is thus an excellent location to investigate the role of massive protostars on protocluster evolution. We observed the ammonia (1,1) and (2,2) inversion transitions with the VLA. Ammonia is the best thermometer for dense and cold gas, and the observed transitions have critical densities able to trace the kinematics of the intracluster gaseous medium. The ammonia emission is extended and distributed in two filamentary structures. The starless condensations are colder than the star-forming cores, but the gas temperature across the whole protocluster is higher (by a factor of ~1.3-1.5) than that measured typically in both infrared dark clouds and low-mass protoclusters. The non-thermal contribution to the observed line broadening is at least a factor of 2 larger than the expected thermal broadening even in starless condensations, contrary to the close-to-thermal line widths measured in low-mass quiescent dense cores. The NH3-to-N2H+ abundance ratio is greatly enhanced (a factor of 10) in the pre--stellar core candidates, probably due to freeze-out of most molecular species heavier than He. The more massive and evolved objects likely play a dominant role in the physical properties and kinematics of the protocluster. The high level of turbulence and the fact that the measured core masses are larger than the expected thermal Jeans masses indicate that turbulence likely was an important factor in the initial fragmentation of the parental clump.

DEUTERIUM FRACTIONATION AS AN EVOLUTIONARY PROBE IN MASSIVE PROTOSTELLAR/CLUSTER CORES

Clouds of high infrared extinction are promising sites of massive star/cluster formation. A large number of cloud cores discovered in recent years allows investigation of possible evolutionary sequence among cores in early phases. We have conducted a survey of deuterium fractionation toward 15 dense cores in various evolutionary stages, from high-mass starless cores to ultracompact Hii regions, in the massive star-forming clouds of high extinction, G34.43+0.24, IRAS 18151-1208, and IRAS 18223-1243, with the Submillimeter Telescope (SMT). Spectra of N2H+ (3 - 2), N2D+ (3 - 2), and C18O (2 - 1) were observed to derive the deuterium fractionation of N2H+, Dfrac \equiv N(N2D+)/N(N2H+), as well as the CO depletion factor for every selected core. Our results show a decreasing trend in Dfrac with both gas temperature and linewidth. Since colder and quiescent gas is likely to be associated with less evolved cores, larger Dfrac appears to correlate with early phases of core evolution. Such decreasing trend resembles the behavior of Dfrac in the low-mass protostellar cores and is consistent with several earlier studies in high-mass protostellar cores. We also find a moderate increasing trend of Dfrac with the CO depletion factor, suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces Dfrac. Our findings suggest a general chemical behavior of deuterated species in both low- and high-mass proto-stellar candidates at early stages. In addition, upper limits to the ionization degree are estimated to be within 2 \times 10^-7 and 5 \times 10^-6. The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.

DEUTERIUM FRACTIONATION AS AN EVOLUTIONARY PROBE IN THE INFRARED DARK CLOUD G28.34+0.06

We have observed the J = 3 − 2 transition of N2H+ and N2D+ to investigate the trend of deuterium fractionation with evolutionary stage in three selected regions in the infrared dark cloud (IRDC) G28.34+0.06 with the Submillimeter Telescope and the Submillimeter Array. A comprehensible enhancement of roughly 3 orders of magnitude in deuterium fractionation over the local interstellar D/H ratio is observed in all sources. In particular, our sample of massive star-forming cores in G28.34+0.06 shows a moderate decreasing trend over a factor of 3 in the N(N2D+)/N(N2H+) ratio with evolutionary stage, a behavior resembling that previously found in low-mass protostellar cores. This suggests a possible extension for the use of the N(N2D+)/N(N2H+) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N2H+(3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of 13CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N2H+ and N2D+ emission is associated with extended structures larger than 8'' (∼0.2 pc).

High mass star formation to the extremes: NGC 3603 at high angular resolution in the near-infrared

High angular resolution observations play a decisive role for our understanding of high mass star formation processes, both within our Galaxy and in extragalactic starburst regions. We take the Galactic starburst template NGC 3603 as paradigm and report here on high angular resolution JHKsL′ observations of the enigmatic, highly reddened sources IRS 9A—C in the NGC 3603 region, which were performed with NACO at ESO's Very Large Telescope Yepun. These broad-band imaging data strongly support the classification of IRS 9A-C as high mass protostellar candidates. We also confirm unambiguously the membership of IRS 9A-C with the NGC 3603 region as gas and dust is seen to be stripped off from their circumstellar envelopes by strong stellar winds, originating from the high mass main sequence stars of the nearby OB cluster. The orientation of these gas and dust streamers coincides with that of a very faint, only marginally detected mini-pillar protruding from the adjacent molecular clump NGC 3603 MM 2. The L′ data show extended envelopes around IRS 9A-C and reveal sub-structures therein which are indicative for non-spherically distributed material. It seems obvious that protostellar mass outflows are at work to clear cavities along the polar axes of the central protostar, and / or that circumstellar disks are taking shape.

A jet-like outflow toward the high-mass (proto) stellar object IRAS 18566+0408

We performed interferometric observations of a high-mass protostellar candidate IRAS 18566+0408 in the \nh3 (J,K)=(1,1), (2,2) and (3,3) inversion transitions, the SiO J=2-1 and HCN J=1-0 lines, and the 43 and 87 GHz continuum emission using the VLA and OVRO. The 87 GHz continuum emission reveals two continuum peaks MM-1 and MM-2 along a molecular ridge. The dominant peak MM-1 coincides with a compact emission feature at 43 GHz, and arises mostly from the dust emission. For dust emissivity index $\beta$ of 1.3, the masses in the dust peaks amount to 70 \msun for MM-1, and 27 \msun for MM-2. Assuming internal heating, the central luminosities of MM-1 and MM-2 are $6 \times 10^4$ and $8 \times 10^3$ \lsun, respectively. The SiO emission reveals a well collimated outflow emanating from MM-1. The jet-like outflow is also detected in \nh3 at velocities similar to the SiO emission. The outflow, with a mass of 27 \msun, causes significant heating in the gas to temperatures of 70 K, much higher than the temperature of $\lsim 15$ K in the extended core. Compact ($< 3''$) and narrow line ($<1.5$ \kms-1) \nh3 (3,3) emission features are found associated with the outflow. They likely arise from weak population inversion in \nh3 similar to the maser emission. Toward MM-1, there is a compact \nh3 structure with a linewidth that increases from 5.5 FHWM measured at 3$''$ resolution to 8.7 \kms-1 measured at 1$''$ resolution. This linewidth is much larger than the FWHM of $<$ 2 in the entire core, and does not appear to originate from the outflow. This large linewidth may arise from rotation/infall, or relative motions of unresolved protostellar cores.

MOLECULAR CLOUDS WITH PECULIAR VELOCITY IN THE OUTER LOCAL ARM

We conducted an analysis of a selected region from the FCRAO <TEX>$^{12}CO$</TEX> Outer Galaxy Survey. The selected region is located between galactic longitude <TEX>$117^{\circ}$</TEX> and <TEX>$124^{\circ}$</TEX> with the velocity of -23 km <TEX>$s^{-1}<V_{LSR}<-10\;km\;s^{-1}$</TEX>. Molecular clouds in this region show a peculiar velocity field, protruding from the Local Arm population. The selected region is divided into 7 clouds by spatial location. Though we were not able to identify the direct driving source for peculiar velocity of our target region, we find that there are several internal YSOs or star forming activities; there are many associated sources like an outflows, a high-mass protostellar candidate and <TEX>$H_2O$</TEX> maser sources. We attribute the driving energy source to older generation of episodic star formation. Masses of main clouds(cloud 1-4) estimated using a conversion factor from <TEX>$^{12}CO$</TEX> luminosity are larger than <TEX>$10^4M_{\odot}$</TEX>. Other components have a small mass as about <TEX>$10^3M_{\odot}$</TEX>. Among main clouds, cloud 2 and 4 seem to be marginally gravitational bound systems as their ratio of <TEX>$M_{CO}$</TEX> to <TEX>$M_{VIR}$</TEX> is about <TEX>$2{\sim}3$</TEX>, and the internal velocity dispersion is larger than the centroid velocity dispersion. Total mass estimated using a conversion factor from <TEX>$^{12}CO$</TEX> luminosity is <TEX>$7.9{\times}10^4M_{\odot}$</TEX>.

Searching for massive pre-stellar cores through observations of N$_\mathsf{2}$H+and N$_\mathsf{2}$D+

Aims. We have measured the deuterium fractionation and the CO depletion factor (ratio between expected and observed CO abundance) in a sample of high-mass protostellar candidates, in order to understand whether the earliest evolutionary stages of high-mass stars have chemical characteristics similar to those of low-mass ones. It has been found that low-mass starless cores on the verge of star formation have large values both of the column density ratio $N({\rm N_2D^+})/N({\rm N_2H^+})$ and of the CO depletion factor. Methods. With the IRAM-30 m telescope and the JCMT we have observed two rotational lines of N 2 H + and N 2 D + , the (2–1) line of C 17 O and DCO + , and the sub-millimeter continuum towards a sample of 10 high-mass protostellar candidates. Results. We have detected N 2 D + emission in 7 of the 10 sources of our sample, and found an average value $N({\rm N_2D^+})/N({\rm N_2H^+})\sim 0.015$. This value is ~3 orders of magnitude larger than the interstellar D/H ratio, indicating the presence of cold and dense gas, in which the physical-chemical conditions are similar to those observed in low-mass pre-stellar cores. The integrated CO depletion factors show that in the majority of the sources the expected CO abundances are larger than the observed values, with a median ratio of 3.2. Conclusions. In principle, the cold gas that generates the N 2 D + emission can be the remnant of the massive molecular core in which the high-mass (proto-)star was born, not yet heated up by the central object. If so, our results indicate that the chemical properties of the clouds in which high-mass stars are born are similar to their low-mass counterparts. Alternatively, this cold gas could be located in one (or more) starless core (cores) near the protostellar object. Due to the poor angular resolution of our data, we cannot distinguish between the two scenarios.

Search for massive protostar candidates in the southern hemisphere

In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with dec < -30 deg, with the SEST antenna at millimeter wavelengths. The sample contains 142 Low sources and 93 High, which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of ~0.5 pc, a mass of ~320 Msolar for a dust temperature Td=30 K, an H_2 density of 9.5E5 cm-3, and a surface density of 0.4 g cm-2. The median values are 0.4 pc, 102 Msolar, 4E4 cm-3, and 0.14 g cm-2, respectively. The mean value of the luminosity-to-mass ratio, L/M ~99 Lsolar/Msolar, suggests that the sources are in a young, pre-ultracompact HII phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or point-like, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is ~3 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above M ~100 Msolar is best fitted with a power-law dN/dM proportional to M-alpha with alpha=2.1, consistent with the Salpeter (1955) stellar IMF, with alpha=2.35.

Observations and Modeling of the 2–25 μm Emission from High‐Mass Protostellar Object Candidates

This is a report on detailed modeling of young high-mass protostellar candidates during their most embedded and obscured phases. We performed narrowband mid-infrared imaging of three candidate high-mass protostellar objects in G11.94-0.62, G29.96-0.02, and G45.07+0.13 at Gemini Observatory using the Thermal-Region Camera and Spectrograph (T-ReCS). The sources were imaged through up to 11 narrowband filters, sampling their SEDs over the entire 2-25 μm infrared range. For the first time, we have fit the observed SEDs of massive protostars with models that take into account departures from spherical symmetry in the infalling envelopes. In this way, we have been able to derive from the models the detailed physical parameters for these earliest stages of massive stellar life. Our detailed modeling suggests that massive star formation can proceed in a way very similar to the formation of low-mass stars.

Search for massive protostellar candidates in the southern hemisphere

We have observed two rotational transitions of both CS and C 17 O, and the 1.2 mm continuum emission towards a sample of 130 high-mass protostellar candidates with δ δ ≥ $-30^{\circ}$, we have selected from the IRAS Point Source Catalogue 429 sources which potentially are compact molecular clouds on the basis of their IR colours. The sample has then been divided into two groups according to the colour indices [25–12] and [60–12]: the 298 sources with [25–12] ≥ 0.57 and [60–12] ≥ 1.30 have been called High sources, the remaining 131 have been called Low sources. In this paper, we check the association with dense gas and dust in 130 Low sources. We have obtained a detection rate of ~$85\%$ in CS, demonstrating a tight association of the sources with dense molecular clumps. Among the sources detected in CS, ~$76\%$ have also been detected in C 17 O and ~$93\%$ in the 1.2 mm continuum. Millimeter-continuum maps show the presence of clumps with diameters in the range $0.2{-}2$ pc and masses from a few $M_{\odot}$ to $10^{5}~M_{\odot}$; H 2 volume densities computed from CS line ratios lie between ~10 4.5 and 10 5.5 cm -3 . The bolometric luminosities of the sources, derived from IRAS data, are in the range $10^{3}{-}10^{6}~L_{\odot}$, consistent with embedded high-mass objects. Based on our results and those found in the literature for other samples of high-mass young stellar objects, we conclude that our sources are massive objects in a very early evolutionary stage, probably prior to the formation of an Hii region. We propose a scenario in which High and Low sources are both made of a massive clump hosting a high-mass protostellar candidate and a nearby stellar cluster. The difference might be due to the fact that the 12 μ m IRAS flux, the best discriminant between the two groups, is dominated by the emission from the cluster in Lows and from the massive protostellar object in Highs .

The Formation of Massive Stars. I. High‐Resolution Millimeter and Radio Studies of High‐Mass Protostellar Candidates

We used the Owens Valley Millimeter Array and the Very Large Array to obtain interferometric maps at millimeter and centimeter wavelengths in both the continuum and various lines [HCO+ (1-0), H13CO+ (1-0), SiO (v = 0, J = 2-1) and H13CN (1-0)] toward a sample of 11 high-mass protostellar candidates. These sources are known from a previous study to be associated with dense gas and dust and to not be associated with H II regions. All 11 sources were detected in HCO+ (1-0), nine in the millimeter continuum and five (of eight observed) in the centimeter continuum. The derived physical parameters confirm the high-mass nature of these molecular clumps and suggest they are gravitationally bound. Molecular outflows were detected toward six sources, with flow masses and momenta much higher than in low-mass young stellar objects. In many of the sources the molecular emission is organized in substructures, resolved both spatially and in velocity. We find that the sources may be characterized by their degree of fragmentation, turbulence, and outflow activity, with the sample dividing into two groups: group 1 cores have multiple peaks but with a clearly dominant component and larger line widths and are systematically associated with outflows, while group 2 cores have several comparable subentities, smaller line widths, and no association with outflows. We speculate that more massive cores may form from smaller cores via coalescence or competitive accretion. Even conservative estimates of outflow mass-loss rates, however, indicate that accretion is the dominant process in the later formation of massive protostars from such cores. We find a flattening of the outflow mass spectra with increasing flow velocities, at variance with previous studies that suggest a steepening with increasing flow velocities. In the light of this result we suggest a reevaluation of the wide-angle wind momentum-driven flow models to describe the acceleration of outflows in the earliest stages of massive star formation.

Search for CO Outflows toward a Sample of 69 High-Mass Protostellar Candidates: Frequency of Occurrence

A survey for molecular outflows was carried out by mapping the CO J = 2-1 line toward a sample of 69 luminous IRAS point sources. Sixty objects have IRAS luminosities from 103 to 105 L☉ and are associated with dense gas traced by NH3, identifying them as high-mass star-forming regions. Among 69 sources, 65 sources have data that are suitable for outflow identification. Thirty-nine regions show spatially confined high-velocity wing emission in CO, indicative of molecular outflows. Most objects without identifiable outflows lie within 0° < l < 50° where outflow signatures are confused by multiple cloud components along the line of sight. Excluding 26 sources with 0° < l < 50°, we found 35 outflows out of 39 sources, which yields an outflow detection rate of 90%. Many of the outflows contain masses of more than 10 M☉ and have momenta of a few hundred M☉ km s-1, at least 2 orders of magnitude larger than those in typical low-mass outflows. This class of massive and energetic outflows is most likely driven by high-mass young stellar objects. The high detection rate indicates that molecular outflows are common toward high-mass young stars. Given the connection between outflows and accretion disks in low-mass stars, we suggest that high-mass stars may form via an accretion-outflow process, similar to their low-mass counterparts.