Young High-mass Stars Research Articles

SMA MILLIMETER OBSERVATIONS OF HOT MOLECULAR CORES

We present Submillimeter Array observations, in the 1.3 mm continuum and the CH_3CN(12-11) line of 17 hot molecular cores associated with young high-mass stars. The angular resolution of the observations ranges from 1".0 to 4".0. The continuum observations reveal large (>3500 AU) dusty structures with gas masses from 7 to 375 Msun, that probably surround multiple young stars. The CH_3CN line emission is detected toward all the molecular cores at least up to the K=6-component and is mostly associated with the emission peaks of the dusty objects. We used the multiple K-components of the CH_3CN and both the rotational diagram method and a simultaneous synthetic LTE model with the XCLASS program to estimate the temperatures and column densities of the cores. For all sources, we obtained reasonable fits from XCLASS by using a model that combines two components: an extended and warm envelope, and a compact hot core of molecular gas, suggesting internal heating by recently formed massive stars. The rotational temperatures lie in the range of 40-132 K and 122-485 K for the extended and compact components, respectively. From the continuum and CH_3CN results, we infer fractional abundances from 10^{-9} to 10^{-7} toward the compact inner components, that increase with the rotational temperature. Our results agree with a chemical scenario in which the CH_3CN molecule is efficiently formed in the gas phase above 100-300 K, and its abundance increases with temperature.

Kinematics of the inner thousand AU region around the young massive star AFGL 2591–VLA3: a massive disk candidate?

[Context] Recent detections of disks around young high-mass stars support the idea of massive star formation through accretion rather than coalescence, but the detailed kinematics in the equatorial region of the disk candidates is not well known, which limits our understanding of the accretion process. [Aims] This paper explores the kinematics of the gas around a young massive star with millimeter-wave interferometry to improve our understanding of the formation of massive stars though accretion. [Methods] We use Plateau de Bure interferometric images to probe the environment of the nearby (~1 kpc) and luminous (~20000 Lsun) high-mass (10-16 Msun) young star AFGL 2591-VLA3 in continuum and in lines of HDO, H218O and SO2 in the 115 and 230 GHz bands. Radiative transfer calculations are employed to investigate the kinematics of the source. [Results] At ~0.5" (500 AU) resolution, the line images clearly resolve the velocity field of the central compact source (diameter of ~ 800 AU) and show linear velocity gradients in the northeast-southwest direction. Judging from the disk-outflow geometry, the observed velocity gradient results from rotation and radial expansion in the equatorial region of VLA3. Radiative transfer calculations suggest that the velocity field is consistent with sub-Keplerian rotation plus Hubble-law like expansion. The line profiles of the observed molecules suggest a layered structure, with HDO emission arising from the disk mid-plane, H218O from the warm mid-layer, and SO2 from the upper disk. [Conclusions] We propose AFGL 2591-VLA3 as a new massive disk candidate, with peculiar kinematics. The rotation of this disk is sub-Keplerian, probably due to magnetic braking, while the stellar wind may be responsible for the expansion of the disk. The expansion motion [...]

Masers as evolutionary tracers of high-mass star formation

AbstractDetermining an evolutionary clock for high-mass star formation is an important step towards realising a unified theory of star formation, as it will enable qualitative studies of the associated high-mass stars to be executed. We have carried out detailed studies of a large number of sources suspected of undergoing high-mass star formation and have found that common maser transitions offer the best opportunity to determine an evolutionary scheme for these objects. We have investigated the relative evolutionary phases of massive star formation associated with the presence or absence of combinations of water, methanol and main-line hydroxyl masers. The locations of the different maser species have been compared with the positions of 1.2 mm dust clumps, radio continuum, GLIMPSE point sources and Extended Green Objects. Comparison between the characteristics of coincident sources has revealed strong evidence for an evolutionary sequence for the different maser species in high-mass star formation regions. We present our proposed sequence for the presence of the common maser species associated with young high-mass stars and highlight recent advances. We discuss future investigations that will be made in this area by comparing data from the Methanol Multibeam (MMB) Survey with chemical clocks from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) Survey.

Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds

(Abridged) We aim to determine the degrees of CO depletion, deuterium fractionation, and ionisation in a sample of seven massive clumps associated with IRDCs. The APEX telescope was used to observe the C17O(2-1), H13CO+(3-2), DCO+(3-2), N2H+(3-2), and N2D+(3-2) transitions towards the clumps. The CO molecules do not appear to be significantly depleted in the observed clumps. The DCO+/HCO+ and N2D+/N2H+ column density ratios are about 0.0002-0.014 and 0.002-0.028, respectively. The former ratio is found to decrease as a function of gas kinetic temperature. A simple chemical analysis suggests that the lower limit to the ionisation degree is in the range x(e)~10^{-8}-10^{-7}, whereas the estimated upper limits range from a few 10^{-6} up to ~10^{-4}. Lower limits to x(e) imply the cosmic-ray ionisation rate of H2 to lie between zeta_H2~10^{-17}-10^{-15} s^{-1}. These are the first estimates of x(e) and zeta_H2 towards massive IRDCs reported so far. The finding that CO is not depleted in the observed sources conforms to the fact that they show evidence of star formation activity which is believed to release CO from the icy grain mantles back into the gas phase. The observed degree of deuteration is lower than in low-mass starless cores and protostellar envelopes. Decreasing deuteration with increasing temperature is likely to reflect the clump evolution. On the other hand, the association with young high-mass stars could enhance zeta_H2 and x(e) above the levels usually found in low-mass star-forming regions. On the scale probed by our observations, ambipolar diffusion cannot be a main driver of clump evolution unless it occurs on timescales >>10^6 yr.

A DETAILED STUDY OF THE ACCRETION DISK SURROUNDING THE HIGH-MASS PROTOSTAR NGC 7538 S

We present deep high angular resolution observations of the high-mass protostar NGC 7538S, which is in the center of a cold dense cloud core with a radius of 0.5 pc and a mass of ~2,000 Msun. These observations show that NGC 7538S is embedded in a compact elliptical core with a mass of 85 - 115 Msun. The star is surrounded by a rotating accretion disk, which powers a very young, hot molecular outflow approximately perpendicular to the rotating accretion disk. The accretion rate is very high, ~ 1.4 - 2.8 10^-3 Msun yr^-1. Evidence for rotation of the disk surrounding the star is seen in all largely optically thin molecular tracers, H13CN J = 1-0, HN13C J = 1-0, H13CO+ J = 1-0, and DCN J = 3-2. Many molecules appear to be affected by the hot molecular outflow, including DCN and H13CO+. The emission from CH3CN, which has often been used to trace disk rotation in young high-mass stars, is dominated by the outflow, especially at higher K-levels. Our new high-angular resolution observations show that the rotationally supported part of the disk is smaller than we previously estimated. The enclosed mass of the inner, rotationally supported part of the disk (D ~ 5", i.e 14,000 AU) is ~ 14 - 24 Msun.

The Herschel Reference Survey

ABSTRACTThe Herschel Reference Survey is a Herschel guaranteed time key project and will be a benchmark study of dust in the nearby universe. The survey will complement a number of other Herschel key projects including large cosmological surveys that trace dust in the distant universe. We will use Herschel to produce images of a statistically-complete sample of 323 galaxies at 250, 350, and 500 μm. The sample is volume-limited, containing sources with distances between 15 and 25 Mpc and flux limits in the K band to minimize the selection effects associated with dust and with young high-mass stars and to introduce a selection in stellar mass. The sample spans the whole range of morphological types (ellipticals to late-type spirals) and environments (from the field to the center of the Virgo Cluster) and as such will be useful for other purposes than our own. We plan to use the survey to investigate (i) the dust content of galaxies as a function of Hubble type, stellar mass, and environment; (ii) the connection between the dust content and composition and the other phases of the interstellar medium; and (iii) the origin and evolution of dust in galaxies. In this article, we describe the goals of the survey, the details of the sample and some of the auxiliary observing programs that we have started to collect complementary data. We also use the available multifrequency data to carry out an analysis of the statistical properties of the sample.

FOUR HIGHLY LUMINOUS MASSIVE STAR-FORMING REGIONS IN THE NORMA SPIRAL ARM. I. MOLECULAR GAS AND DUST OBSERVATIONS

We report molecular line and dust continuum observations, made with the SEST telescope, towards four young high-mass star forming regions associated with highly luminous (L> 6x10^5 Lsun) IRAS sources (15290-5546, 15502-5302, 15567-5236 and 16060-5146). Molecular emission was mapped in lines of CS (J=2-1, 3-2 and 5-4), SiO (J=2-1 and 3-2), CH3OH (Jk=3k-2k and 2k-1k), and C34S (J=3-2). In addition, single spectra at the peak position were taken in the CO, 13CO and C18O (J=1-0) lines. We find that the luminous star forming regions are associated with molecular gas and dust structures with radii of typically 0.5 pc, masses of ~5x10^3 Msun, column densities of ~5x10^{23} cm^{-2}, molecular hydrogen densities of typically ~2x10^5 cm^{-3} and dust temperatures of ~40 K. The 1.2 mm dust continuum observations further indicate that the cores are centrally condensed, having radial density profiles with power-law indices in the range 1.6-1.9. We find that under these conditions dynamical friction by the gas plays an important role in the migration of high-mass stars towards the central core region, providing an explanation for the observed stellar mass segregation within the cores.

The RMS survey

Context: The Red MSX Source (RMS) survey has identified a large sample of candidate massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. Aims: To search for H2O masers towards a large sample of young high mass stars and to investigate the statistical correlation of H2O masers with the earliest stages of massive star formation. Methods: We have used the Mopra Radio telescope to make position-switched observations towards ~500 UCHII regions and MYSOs candidates identified from the RMS survey and located between 190\degr < l < 30\degr. These observations have a 4$\sigma$ sensitivity of ~1 Jy and a velocity resolution of ~0.4 km/s.} Results: We have detected 163 H2O masers, approximately 75% of which were previously unknown. Comparing the maser velocities with the velocities of the RMS sources, determined from 13CO observations, we have identified 135 RMS-H2O maser associations, which corresponds to a detection rate of ~27%. Taking into account the differences in sensitivity and source selection we find our detection rate is in general agreement with previously reported surveys. Conclusions: We find similar detection rates for UCHII regions and MYSOs candidates, suggesting that the conditions needed for maser activity are equally likely in these two stages of the star formation process. Looking at the detection rate as a function of distance from the Galactic centre we find it significantly enhanced within the solar circle, peaking at ~37% between 6-7 kpc, which is consistent with previous surveys of UC HII regions, possibly indicating the presence of a high proportion of more luminous YSOs and HII regions.

The nature of the methanol maser ring G23.657-00.127

Methanol masers are associated with young high-mass stars and are an important tool for investigating the process of massive star formation. The recently discovered methanol maser ring in G23.657-00.127 provides an excellent ``laboratory'' for a detailed study of the nature and physical origin of methanol maser emission, as well as parallax and proper motion measurements. Multi-epoch observations of the 12.2 GHz methanol maser line from the ring were conducted using the Very Long Baseline Array. Interferometric observations with milliarcsecond resolution enabled us to track single maser spots in great detail over a period of 2 years. We have determined the trigonometric parallax of G23.657-00.127 to be 0.313+/-0.039 mas, giving a distance of 3.19{+0.46}{-0.35} kpc. The proper motion of the source indicates that it is moving with the same circular velocity as the LSR, but it shows a large peculiar motion of about 35 km/s toward the Galactic center.

THE COLLIMATED JET SOURCE IN IRAS 16547-4247: TIME VARIATION, POSSIBLE PRECESSION, AND UPPER LIMITS TO THE PROPER MOTIONS ALONG THE JET AXIS

The triple radio source detected in association with the luminous infrared source IRAS 16547-4247 has previously been studied with high angular resolution and high sensitivity with the Very Large Array at 3.6 cm wavelength. In this paper, we present new 3.6 cm observations taken 2.68 years after the first epoch that allow a search for variability and proper motions, as well as the detection of additional faint sources in the region. We do not detect proper motions along the axis of the outflow in the outer lobes of this source at a 4σ upper limit of ∼160 km s−1. This suggests that these lobes are probably working surfaces where the jet is interacting with a denser medium. However, the brightest components of the lobes show evidence of precession, at a rate of 008 yr−1 clockwise in the plane of the sky. It may be possible to understand the distribution of almost all the identified sources as the result of ejecta from a precessing jet. The core of the thermal jet shows significant variations in flux density and morphology. We compare this source with other jets in low- and high-mass young stars and suggest that the latter can be understood as a scaled-up version of the former.

MWC 297: a young high-mass star rotating at critical velocity

MWC 297 is a young massive nearby B[e] star. The central star has a large projected rotational velocity of 350 km/s. Despite the wealth of published observations, the nature of this object and its dust-rich surroundings is not well understood. With the present paper, we shed light on the geometrical structure of the circumstellar matter which produces the near- to mid-infrared flux excess, and construct an overall image of the source's appearance and evolutionary status. The H-, K- and N-band brightness distribution of MWC 297 is probed with the ESO interferometric spectrographs AMBER and MIDI. We have obtained visibility measurements on 3 AMBER and 12 MIDI baselines, covering a wide range of spatial frequencies. We have reconstructed the brightness distribution in H, K and N with a geometric model consisting of three Gaussian disks with different extent and brightness temperature. This model can account for the entire near- to mid-IR emission of MWC 297. The near- and mid-IR emission, including the silicate emission at 10 micron, emanates from a very compact region (FWHM < 1.5 AU) around the central star. We argue that the circumstellar matter in the MWC 297 system is organized in a disk, seen under moderate (i < 40 deg) inclination. The disk displays no inner emission-free gap at the resolution of our interferometric data. The low inclination of the disk implies that the actual rotational velocity of the star exceeds its critical velocity. We discuss the impact of this result in terms of the formation of high-mass stars, and the main-sequence evolution of classical Be stars.

Multiple Sources toward the High-Mass Young Star S140 IRS 1

S140 IRS 1 is a remarkable source where the radio source at the center of the main bipolar molecular outflow in the region is elongated perpendicular to the axis of the outflow, an orientation opposite to that expected if the radio source is a thermal jet exciting the outflow. We present results of 1.3 cm continuum and H2O maser emission observations made with the Very Large Array in its A configuration toward this region. In addition, we also present results of continuum observations at 7 mm and reanalyze observations at 2, 3.5, and 6 cm (previously published). IRS 1A is detected at all wavelengths, showing an elongated structure. Three water maser spots are detected along the major axis of the radio source IRS 1A. We have also detected a new continuum source at 3.5 cm (IRS 1C) located ≃0.6'' northeast of IRS 1A. The presence of these two young stellar objects (IRS 1A and 1C) could explain the existence of the two bipolar molecular outflows observed in the region. In addition, we have also detected three continuum clumps (IRS 1B, 1D, and 1E) located along the major axis of IRS 1A. We discuss two possible models to explain the nature of IRS 1A: a thermal jet and an equatorial wind.

AGLIMPSE-based search for 6.7-GHz methanol masers and the lifetime of their spectral features

The University of Tasmania Mt Pleasant 26-m and Ceduna 30-m radio telescopes have been used to search for 6.7-GHz class II methanol masers towards two hundred GLIMPSE sources. The target regions were selected on the basis of their mid-infrared colours as being likely to be young high-mass star formation regions and are either bright at 8.0 μm, or have extreme [3.6]-[4.5] colour. Methanol masers were detected towards 38 sites, nine of these being new detections. The prediction was that approximately 20 new 6.7-GHz methanol masers would be detected within 3.5 arcmin of the target GLIMPSE sources, but this is the case for only six of the new detections. A number of possible reasons for the discrepancy between the predicted and actual number of new detections have been investigated. It was not possible to draw any firm conclusions as to the cause, but it may be because many of the target sources are at an evolutionary phase prior to that associated with 6.7-GHz methanol masers. Through comparison of the spectra collected as part of this search with those in the literature, the average lifetime of individual 6.7-GHz methanol maser spectral features is estimated to be around 150 years, much longer than is observed for 22-GHz water masers.

The infrared environments of masers associated with star formation

AbstractThe near infrared (1–2μm) and the thermal infrared (3–25μm) trace many of the environments in which masers are thought to reside, including shocks, outflows, accretion disks, and the dense medium near protostars. After a number of recent surveys it has been found that there is a higher detection rate of mid-IR emission towards masers than cm radio continuum emission from UC HII regions, and that the mid-IR emission is actually more closely cospatial to the maser locations. A high percentage of water and methanol masers that are not coincident with the UC HII regions in massive star forming regions are likely to be tracing outflows and extremely young high mass stars before the onset of the UC HII region phase. After a decade of groundwork supporting the hypothesis that linearly distributed class II methanol masers may generally trace accretion disks around young massive stars, compelling evidence is mounting that these masers may generally be associated with outflows instead. Substantiation of this claim comes from recent outflow surveys and high angular resolution mid-IR imaging of the maser environments.

Photon Bubbles in the Circumstellar Envelopes of Young Massive Stars

We show that the optically-thick dusty envelopes surrounding young high-mass stars are subject to the photon bubble instability. The infrared radiation passing through the envelope amplifies magnetosonic disturbances, with growth rates in our local numerical radiation MHD calculations that are consistent with a linear analysis. Modes with wavelengths comparable to the gas pressure scale height grow by more than two orders of magnitude in a thousand years, reaching non-linear amplitudes within the envelope lifetime. If the magnetic pressure in the envelope exceeds the gas pressure, the instability develops into trains of propagating shocks. Radiation escapes readily through the low-density material between the shocks, enabling accretion to continue despite the Eddington limit imposed by the dust opacity. The supersonic motions arising from the photon bubble instability can help explain the large velocity dispersions of hot molecular cores, while conditions in the shocked gas are suitable for maser emission. We conclude that the photon bubble instability may play a key role in the formation of massive stars.

Inferences from the kinematic properties of 6.7 GHz methanol masers

Context. It is now well established that the strong and widespread 6.7 GHz methanol masers are associated with young high mass stars. A still unsolved question is where in the circumstellar environment the masers arise. Aims. We address this question by considering an ensemble of rest frame maser velocities of 337 maser features. Methods. The CS(2-1) spectra of 63 methanol maser sources were used to derive systemic velocities and velocity dispersion of the thermal gas. Using the systemic velocities and the velocities of the 337 maser features in the 63 sources, a single distribution of rest frame maser velocities was constructed. This distribution as well as other kinematic information about the masers are used to evaluate four proposed scenarios for where the masers might arise in the circumstellar environment. Results. It is shown that kinematically the masers are not associated with hot cores. We also argue that the scenario in which the masers are associated with an external generated planar shock that propagates into a rotating core cannot explain the observed kinematic properties of the masers. It was found that a simple Keplerian-like disk model is consistent with the observed distribution of rest frame maser velocities. Although outflows have the potential to explain the data, it was not possible to fully test this possibility due to the diverse nature of outflows. Conclusions.

A survey of OH masers towards high mass protostellar objects

Context. Masers are important tracers of the early evolution of young high mass stars, but the relationship between different types of maser and the evolutionary state of the exciting source remains unclear. Aims. To determine whether OH masers are common towards candidate high mass protostellar objects. Methods. We present a survey of OH maser emission towards a sample of high mass protostellar objects made using the Nancay and GBT telescopes. Results. OH maser emission was detected towards 63 objects with 36 new detections. There are 56 star-forming regions and 7 OH/IR candidates. Nearly half of the detected sources have OH flux densities ≤1 Jy. There is no evidence that sources with OH masers have a different range of luminosities from the non-maser sources. The results of this survey are compared with previous H 2 O and class II CH 3 OH maser observations of the same objects. Some of the detected sources are only associated with OH masers and some sources are only associated with the 1720 MHz OH maser line. The velocity range of the maser emission suggests that the water maser sources may be divided into two groups. The detection rates and velocity range of the OH and Class II CH 3 OH masers support the idea that there is a spatial association of the OH and Class II CH 3 OH masers. The sources span a wide range in R, the ratio of the methanol maser peak flux to OH 1665 MHz maser peak flux, however there are only a few sources with intermediate values of R, 8 < R < 32, which has characterised previous samples. The majority of the sources are either methanol-favoured or OH-favoured. Sources which have masers of any species, OH, water or methanol, have redder [100 μm-12 μm] IRAS colours than those without masers. However, there is no evidence for different maser species tracing different stages in the evolution of these young high mass sources. Conclusions. The detection of OH masers towards 26% of a sample of 217 sources should remove any doubt about the existence of OH maser emission towards these objects or this early evolutionary stage. Previous observations which have shown that the OH maser emission from similar sources traces the circumstellar disks around the objects. This combined with the sensitivity of the OH emission to the magnetic field, make the newly detected sources interesting candidates for future follow-up at high angular resolution.

The critical role of disks in the formation of high-mass stars

Although massive stars (commonly defined as those in excess of about eight solar masses, or with initial luminosities of a thousand times the solar luminosity or more) have an enormous impact on the galactic environment, how they form has been a mystery. The solution probably involves the existence of accretion disks. Rotational motions have been found in the gas surrounding young high-mass stars, which suggests that non-spherical accretion could be the fundamental ingredient of the massive-star formation recipe.

Molecular outflows from G35.2-0.74N

We present interferometric observations of the massive star-forming region associated with G35.2-0.74N using the Berkeley Illinois Maryland Association Array. With the aim of better understanding the outflow in this region we observed , and the 2.7 mm continuum. The and continuum emission peak close to the sources G35.2-0.74N (G35.2N) and G35MM2 and indicate a mass of ~ of circumstellar material associated with these sources. The 12CO traces a weak filament of emission coincident with the radio and infrared jet from G35.2N but the emission is dominated by an extended outflow with a NE-SW axis which has a total mass of ~13 . Each lobe of this extended outflow has a hollow shell structure and the location of these shells makes the source G35MM2 a more likely candidate for the source driving the outflow than G35.2N. The mass-velocity distribution is calculated for several parts of the outflow. Fitting these distributions with power laws some of the same break-points are seen as previously identified in the emission from the outflow. We conclude this indicates the temperature dependence of emissivity is not responsible for all the break-points seen and molecular dissociation is a more plausible explanation for their origin. We model the molecular outflow using the ZEUS-2D hydrodynamic code which we have augmented so that it can also track the composition of the gas. We find that a general hydromagnetic wind, without an enchanced, on axis, jet-like component, can reproduce the shape of the observed outflow. Models looking at the time evolution of the stellar wind indicate that the structure of the outflow is dominated by the initial wind conditions, rather than its later evolution. The models also show that scaling the density of the wind profile effects the apparent collimation of the resulting outflow. This may help explain some of the apparent differences between outflows from high mass and low mass young stars.

The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611

An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmosphere calculations have been used to estimate effective temperatures (from either the helium spectrum or the silicon ionization equilibrium) and gravities (from the hydrogen spectrum). Projected rotational velocities have been deduced from the helium spectrum (for fast and moderate rotators) or the metal line spectrum (for slow rotators). The origin of the low gravity estimates for apparently near main sequence objects is discussed and is related to the stellar rotational velocity. The atmospheric parameters have been used to estimate cluster distances (which are generally in good agreement with previous determinations) and these have been used to estimate stellar luminosities and evolutionary masses. The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed. Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters. Projected rotational velocities for targets in the older clusters, NGC 3293 and NGC 4755, have been found to be systematically larger than those for the field, confirming recent results in other similar age clusters. The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities. For the relatively unevolved targets in the older clusters, NGC 3293 and NGC 4755, the peak of the velocity distribution would be 250 km s −1 with a full-width-half-maximum of approximately 180 km s −1 . For NGC 6611, the sample size is relatively small but implies a lower mean rotational velocity. This may be evidence for the spin-down effect due to angular momentum loss through stellar winds, although our results are consistent with those found for very young high mass stars. For all three clusters we deduce present day mass functions with Γ-values in the range of −1.5 to −1.8, which are similar to other young stellar clusters in the Milky Way.