Young High-mass Stars Research Articles

2.3 μm CO emission and absorption from young high-mass stars in M 17

Aims. We are studying the extremely young cluster of M 17 to investigate the birth of high-mass stars and the initial mass function. Methods. Deep JHKL imaging and K-band spectroscopy from the VLT of 201 stars toward the cluster is presented. Results. The majority of 104 stars show the CO band-head in absorption. Half of them emit X-rays and/or have infrared excess, indicative of very young objects. Their intrinsic IR luminosity is compatible with intermediate and high-mass pre-main sequence stars. Nine additional stars have the CO feature in emission, while sixty sources are lacking any stellar spectral feature due to veiling by circumstellar dust. Conclusions. We suggest that CO absorption is - as in the case of low-mass stars - also a common feature during the early evolution of stars with higher masses. According to model calculations the observed CO absorption is most likely a sign of heavily accreting protostars with mass accretion rates above 10 -5 Moyr -1 .

Anatomy of the S255–S257 complex – triggered high-mass star formation

AbstractWe present a multi-wavelength (NIR to radio) and multi-scale (1 AU to 10 pc) study of the S255–S257 complex of young high-mass (proto)stars. The complex consists of two evolved HII regions and a molecular gas filament in which new generations of high mass stars form. Four distinct regions are identified within this dusty filament: a young NIR/optical source cluster, a massive protostar binary, a (sub)millimetre continuum and molecular clump in global collapse and a reservoir of cold gas. Interestingly, the massive binary protostellar system is detected through methanol maser and mid-IR emission at the interface between the NIR cluster and the cold gas filament. The collapsing clump is located to the north of the NIR cluster and hosts a young high-mass star associated with an outflow that is observed in mid-IR, methanol maser and radio emission. We interpret this anatomy as the possible result of triggered star formation, starting with the formation of two HII regions, followed by the compression of a molecular gas filament in which a first generation of high-mass stars forms (the NIR cluster), which then triggers the formation of high mass protostars in its near environment (the massive protostellar binary). The global collapse of the northern clump might be due to both the expansion of the HII regions that squashes the filament. In conclusion, we witness the formation of four generations of clusters of high-mass stars in S255–S257.

Infrared Dark Clouds: Precursors to Star Clusters

Infrared Dark Clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs. IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 Msun, with a median mass of ~940 Msun. Each IRDC contains at least one compact (<=0.5 pc) dust core and most show multiple cores. We find 140 cold millimeter cores unassociated with MSX 8um emission. The core masses range from 10 to 2,100 Msun, with a median mass of ~120 Msun. The slope of the IRDC core mass spectrum (alpha ~ 2.1 +/- 0.4) is similar to that of the stellar IMF. Assuming that each core will form a single star, the majority of the cores will form OB stars. IRDC cores have similar sizes, masses, and densities as hot cores associated with individual, young high-mass stars, but they are much colder. We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation. In addition, because IRDCs contain many compact cores, and have the same sizes and masses as molecular clumps associated with young clusters, we suggest that IRDCs are the cold precursors to star clusters. Indeed, an estimate of the star formation rate within molecular clumps with similar properties to IRDCs (~2 Msun/yr) is comparable to the global star formation rate in the Galaxy, supporting the idea that all stars may form in such clumps.

Imaging and integral-field spectroscopy of shocked H2 around G25.65+1.05

Near-infrared imaging of the emission from molecular hydrogen is a powerful method for discovering outflows in star-forming regions. We present new near-infrared images, and long-slit and integral-field spectroscopy of the ultracompact HII region G25.65+1.05. These new observations reveal shocked H 2 emission associated with a bipolar outflow from a young high-mass star at the centre of the source. The physical parameters of the outflow are discussed and compared with those of outflows from lower mass stars.

The ‘missing’ young stellar objects in the central parsec of the Galaxy: evidence for star formation in a massive accretion disc and a top-heavy initial mass function

Abstract A few dozen young high-mass stars orbit Sgr A* at distances as short as ∼0.1 pc, where star formation should be quenched by the strong tidal shear from Sgr A*. The puzzling young stellar population is believed to come into existence in one of two ways: (i) ‘normal’ star formation at several tens of parsecs in a very massive star cluster that then spiralled in; or (ii) star formation in situ in a massive self-gravitating disc. We propose to constrain these two scenarios via the expected X-ray emission from young low-mass stars that should have formed alongside the massive stars. To this end we compare the recent Chandra observations of X-ray emission from young stars in the Orion nebula, and the Chandra observations of the Sgr A* field. We show that the cluster spiral-in model is ruled out irrespectively of the initial mass function (IMF) of the young stars. In addition, for the in situ model, we find that no more than a few thousand low-mass stars could have formed alongside the massive stars. This is more than a factor of 10 fewer than expected if these stars were formed with the standard IMF as elsewhere in the Galaxy. The young stars in the Galactic Centre are thus the first solid observational evidence for star formation in active galactic nucleus (AGN) discs and also require the IMF of these stars to be top-heavy. We briefly consider the implications of these results for AGN in general.

X-ray perspective on young stellar populations in objects with nuclear activity - the case of our Galactic Center-

Although our Galactic Center harbors a black hole (Sgr A*) of a few million solar masses, it and its environments are very quiet at present. In X-rays however, the close vicinity of Sgr A* shows very unique and various phenomena mostly originated from young stellar populations. We report on the X-ray perspective on the young stellar populations which are related to our Galactic Center activities. The discussion is essentially based on the observational facts of new X-ray objects in the Galactic Center region in the area. They are; Clusters of young high mass stars, which are Sgr B2, Arches, IRS 13 and Quintuplet.X-ray reflections in the giant molecular clouds, such as Sgr B2, Sgr C, M0.01-0.09 and others.New candidates of X-ray supernova remnants (SNRs), which are Sgr A East, G0.570-0.018 and G359.8-0.3.Non-thermal Jets, Filaments and Shells, which are unique X-ray features in the GC region. These X-ray features may be closely related with each other, hence may have common origins. A unified picture is presented for the X-ray activity of our Galactic Center comparing with the X-ray spectra from other type of galaxies such as;Star burst galaxy (NGC 253), low luminosity AGN (M 81) and Seyfert 2 (NGC 1068).

Detection of steam in the circumstellar disk around a massive Young Stellar Object

We report on the observation of hot water vapor (steam) in the inner AU of a young massive star located in the star- forming region IRAS 08576-4334. The water lines are detected in a medium resolution (R ∼ 10 000) K-band spectrum taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU. The water vapor is at a mean temperature of 1565 ± 510 K, cooler than the hot CO found in the same object, which is at � 1660 K; and the column density is N(H2O) = (2.5 ± 0.4) × 10 18 cm −2 . The profile of both H2O and CO lines is best reproduced by the emission from a Keplerian disk. To interpret the data, we also investigate the formation of molecules and especially CO and water vapor in the inner hot and dense part of disks around young high mass stars using a pseudo time-dependent gas-phase chemical model. Molecules are rapidly photodissociated but this destruction is compensated for by an efficient formation due to fast neutral-neutral reactions. The ability of CO molecules to self-shield significantly enhances its abundance. Water molecules are sufficiently abundant to be detectable. The observed H2O/CO ratio is reproduced by gas at 1600 K and by an enhanced UV field over gas density ratio IUV/nH = 10 −4 −10 −6 .T he simulations support the presence of CO and H2O molecules in the inner disks around young massive stars despite the strong UV radiation and show that the OH radical plays an essential role in hot gas chemistry.

X-ray studies of massive star birth regions

I report X-ray features of young stage of intermediate and high mass stars. The giant molecular cloud Sagittarius B2 (Sgr B2) exhibits more than dozen X-ray sources, two are associated with the ultra compact (UC) HII complex, Sgr B2 Main. The sources show large absorption of $\gg$ 10 23 Hcm −2 , the largest among any known stellar X-ray sources. The Arches cluster exhibits 3 extremely bright (a few $\times10^{33}$ ergs s −1 ) X-ray sources associated with the infrared (IR) massive stars. The X-ray spectra have 1-3 keV temperature in thin thermal model. Elongated diffuse 6.4 keV line emission is found. The Monoceros R2 cloud exhibits half dozen X-ray sources associated with young high-mass IR stars. They show rapid time variability and a thin thermal spectrum of ${\sim }2$ keV temperature. Among 28 ASC A pointing on intermediate-mass pre-main-sequence stars, or Herbig Ae/Be stars (HAeBes), eleven are found to be plausible X-ray sources. The general X-ray properties of these HAeBes are; (1) the plasma temperature and time variability are higher than those of high mass main-sequence stars, and more similar to low mass stars; (2) the X-ray luminosities come to the upper end of low mass pre-main sequence stars, or in some cases exceed that; (3) the X-ray activity of HAeBe decreases at the age of about a few×10 6 years. Using above observational facts, I propose a unified picture of X-ray activity of young stars in the wide mass range.

Subarcsecond mid-infrared and radio observations of the W3 IRS5 protocluster

Observations at mid-infrared (4.8-17.65 μm) and radio (0.7-1.3 cm) wavelengths are used to constrain the structure of the high-mass star-forming region W3 IRS5 on 0.1 (200 AU) scales. Two bright mid-infrared sources are detected, as well as diffuse emission. The bright sources have associated compact radio emission and probably are young high-mass stars. The measured sizes and estimated temperatures indicate that these sources together can supply the observed far-infrared luminosity. However, an optically thick radio source with a possible mid-infrared counterpart may also contribute significant luminosity; if so, it must be extremely deeply embedded. The infrared colour temperatures of 350-390 K and low radio brightness suggest gravitational confinement of the H II regions and ongoing accretion at a rate of a few 10 -8 M ○. yr -1 or more. Variations in the accretion rate would explain the observed radio variability. The low estimated foreground extinction suggests the existence of a cavity around the central stars, perhaps blown by stellar winds. At least three radio sources without mid-infrared counterparts appear to show proper motions of ∼100 km s -1 , and may be deeply embedded young runaway OB stars, but more likely are clumps in the ambient material which are shock-ionized by the OB star winds.

Collimated molecular jets from high-mass young stars: IRAS 18151-1208

Recent near-IR images of massive star forming regions have revealed two collimated jets in the IRAS 18151-1208 region, one of which is almost a parsec in length (Varricatt et al.). Follow-up high-spectral-resolution echelle spectroscopy and 2-dimensional “integral field” spectroscopy of the associated molecular shock features are presented here. From these data kinematic information and excitation maps are extracted, which show that the two jets are morphologically, kinematically and energetically similar to their counterparts from low mass protostars. The close association between the H2 emission features and the high-velocity CO emission presented by Beuther et al. also suggests that the CO represents gas entrained by these two very collimated jets. From the mass and momentum of the molecular gas, and the luminosity of the H2 features, it is clear that the flows must be powered by massive sources. To all intents and purposes, the molecular jets appear to be scaled-up versions of low-mass YSO jets. Collectively, the observations add further support to the idea that massive stars are formed through vigorous disk accretion, and that, while in their earliest stages of evolution, massive protostars drive collimated jets.

Formation scenarios for the young stellar associations between galactic longitudes l $\mathsf{= 280\degr{-}360\degr}$

We investigate the spatial distribution, the space velocities and age distribution of the pre-main sequence (PMS) stars belonging to Ophiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of the young early-type star members of the Scorpius-Centaurus OB association. These young stellar associations extend over the galactic longitude range from 280 to 360, and are at a distance interval of around 100 and 200 pc. This study is based on a compilation of distances, proper motions and radial velocities from the literature for the kinematic properties, and of basic stellar data for the construction of Hertzsprung-Russel diagrams. Although there was no well-known OB association in Chamaeleon, the distances and the proper motions of a group of 21 B- and A-type stars, taken from the Hipparcos Catalogue, lead us to propose that they form a young association. We show that the young early-type stars of the OB associations and the PMS stars of the SFRs follow a similar spatial distribution, i.e., there is no separation between the low and the high-mass young stars. We find no dierence in the kinematics nor in the ages of these two populations studied. Considering not only the stars selected by kinematic criteria but the whole sample of young early-type stars, the scattering of their proper motions is similar to that of the PMS stars and all the young stars exhibit a common direction of motion. The space velocities of the Hipparcos PMS stars of each SFR are compatible with the mean values of the OB associations. The PMS stars in each SFR span a wide range of ages (from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for Upper Scorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and for Lower Centaurus Crux (LCC). Thus, our results do not confirm that UCL is older than the LCC association. Based on these results and the uncertainties associated with the age determination, we cannot say that there is indeed a dierence in the age of the two populations. We analyze the dierent scenarios for the triggering of large-scale star-formation that have been proposed up to now, and argue that most probably we are observing a spiral arm that passes close to the Sun. The alignment of young stars and molecular clouds and the average velocity of the stars in the opposite direction to the Galactic rotation agree with the expected behavior of star formation in nearby spiral arms.

RXTE observations of the strongly absorbed sources IGR J16318-4848 and IGR J16358-4726

We analyzed the RXTE observations of two strongly absorbed sources, IGR J16318-4848 and IGR J16358-4726. We were able to obtain the 3–25 keV spectra of the sources by taking into account the contribution of the Galactic diffuse background to the X-ray flux recorded with the RXTE/PCA spectrometer. The spectra of the sources are well described by a power-law decrease of the photon flux with energy with a photon index of ∼1 and strong photoabsorption. The photoabsorption column density n H L for IGR J16318-4848 derived from the RXTE observation on March 14.1, 2003, is shown to be much higher than its value obtained by the XMM observatory on February 10.7, 2003. This result may suggest that the source has variable absorption that may depend on the orbital phase of the system. We point out that all of the three X-ray sources discovered by the INTEGRAL observatory in the region (l,b)∼(336,0) (IGR J16318-4848, IGR J16320-4751, and IGR J16358-4726) have strong intrinsic photoabsorption and may be high-mass binaries. Their proximity to the region where the tangent to the Galactic spiral arm passes, i.e., to the region of enhanced concentration of young high-mass stars, can serve as an indirect confirmation of this assumption. If our assumption about the positions of the sources in the Norma spiral arm is valid, then we can roughly estimate their heliocentric distances: ∼6–8 kpc.

A Disk/Jet System toward the High‐Mass Young Star in AFGL 5142

We present observational studies of a high-mass star-forming region AFGL 5142 in the NH3 (J,K) = (1,1), (2,2), (3,3), and (4,4) inversion transitions. We discover a compact structure of 1'' (or 1800 AU) in NH3 emission toward the position of a high-mass young star. The star, with a spectral type of B2 if it is at the zero-age main sequence, excites a weak centimeter continuum source and drives a well-collimated SiO jet. The compact NH3 structure, with a temperature of 70 K, gives rise to a broad NH3 line emission of 6.4 km s-1 (FWHM). The small size of the NH3 feature, broad line emission, and the association with a SiO jet indicate that the object is most likely an unresolved rotating disk. The compact disk is embedded in a flattened core of gas temperatures of about 20 K. The extended core delineates an X shape with the SiO jet located in the void of the dense and cold NH3 gas. A faint component of NH3 emission can be identified, especially in the highly excited NH3 (3,3) and (4,4) lines, tracing the SiO jet. This component, arising from hot gas of about 170 K, is likely heated by shocks in the outflow. The coincidence of a disk and a jet with the radio continuum source toward AFGL 5142 indicates that this high-mass star may have formed via a disk-mediated accretion process.

An Infrared Jet from a High-Mass Young Star

Jets are a direct consequence of accretion in the inner regions of circumstellar disks. They trace the structure of the accretion disk and, indirectly, the star formation mechanism. Here we report on the discovery of a near-infrared jet from a young B1 star, one of the most luminous young stars known to exhibit such a structure. The jet is seen in L' images of IRAS 18556+0136 in the G35.2-0.74N region; a significant fraction of the emission is due to Brα line emission. At shorter wavelengths, the jet is obscured: the colors of the bipolar nebula are consistent with 25 mag visual extinctions to the near lobe and 40 mag to the far lobe. A previously detected radio continuum source and an elongated clump of OH maser spots coincide with a break in the jet. This is interpreted as the location of the circumstellar disk surrounding the embedded young star. A lower limit of ~170 mag is derived for the extinction to the exciting source. This provides an estimate of 0.15 M☉ for the mass of the circumstellar disk within ~1500 AU of the central source. Emission knots on either side of a second, weaker radio continuum source offset ~3'' from the jet source suggests the presence of a second outflow source in the region. The lack of known jets in high-mass protostars, in contrast to their prevalence among low-mass systems, points to a difference in the star formation process, e.g., mergers. The jet from this B1 star suggests that the mechanisms that form low-mass stars can operate up to at least 10 M☉ stars.

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.

Shock-heated NH3 in a Molecular Jet Associated with a High-Mass Young Star.

We present the discovery of shock-excited NH3 in a well-collimated jet associated with the extremely young high-mass star IRAS 20126+4104. The NH3 (3, 3) and (4, 4) emission is dominated by three clumps along the SiO jet. At the end of the jet, there exists strong and broad (+/-10 km s-1) NH3 (3, 3) emission. With typical brightness temperatures greater than 500 K, the overall emission indicates a weakly inverted population and appears in an arc, consistent with the excitation by bow shocks. There are two bright spots in the NH3 (3, 3) emission with brightness temperatures of approximately 2000 K. The narrow line width (1.5 km s-1 FWHM), the small sizes (<0&farcs;3), and the unusually high brightness temperature of the features are indicative of maser emission. Our observations provide clear evidence that NH3 (3, 3) masers are excited in shock regions in molecular outflows.

A Rotating Disk around a High-Mass Young Star

We report the discovery of a disk around the young high-mass star IRAS 20126+4104. In the NH3 (1, 1) and (2, 2) lines, we detected a flattened disklike structure (6''$^{{′}{′}}$ -->$^{{′}{′}}$ --> × 3''$^{{′}{′}}$ -->$^{{′}{′}}$ --> or 10,000 × 5000 AU) roughly perpendicular to a bipolar molecular outflow. A velocity gradient exists along the major axis of the disk. The disk rotates faster toward the center. Our findings provide direct verification that disks exist during the formation of high-mass stars.

A Survey of Dust Features in the 3 Micron Spectra of YSO Candidates

We present the results of spectrophotometry from 1.3 to 4.2 μm of 45 IRAS sources in the list of Campbell, Persson, & Matthews that have been presumed to be luminous young stellar objects because of their infrared colors and large IRAS fluxes. About 70% of the objects show 3.1 μm H2O ice absorption or 3.3 μm unidentified infrared emission, or both. Among those with 3.1 μm H2O ice absorption, three objects seem to have a distinct absorption near 3.4 μm, and their absorption profiles are different than the optical depth profiles of young stellar objects in the literature. In the HKL two-color diagram, objects with ice absorption occupy the same region as previously known young stellar objects with the ice feature, and those with unidentified infrared emission overlap with Herbig Ae/Be stars. We interpret the distribution in the HKL two-color diagram as indicating the evolutionary sequence of intermediate- to high-mass young stars.