D. E. A. Nürnberger

The formation of a massive protostar through the disk accretion of gas.

The formation of low-mass stars like our Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above ten solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via accretion through a disk. Given this uncertainty and the fact that most massive stars are born in dense clusters, it was suggested that high-mass stars are the result of the runaway merging of intermediate-mass stars. Here we report observations that clearly show a massive star being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and the accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.

The Stellar Population of M17

The stellar content of M17 has been investigated by multicolor photometry and spectroscopy. Various independent estimates yield a distance of 2.1 ± 0.2 kpc. The ratio of total-to-selective extinction is R = 3.9. Within a projected area of 3.6 × 3.7 pc, there are several thousand stars. About 74% of them show infrared excess suggesting the presence of dense circumstellar material; the excess frequency is higher for fainter stars. The number of spectroscopically classified exciting stars could be enlarged from 13 to 46. The two central O4 stars are both spectroscopic binaries; multiplicity of other early O-type stars could also be established, increasing the number of high-mass stars even further. Our data suggest at least two episodes of star formation: There are about 500 ZAMS sources (2 < AV < 7)—among them many spectroscopically classified OB stars and a significant fraction of lower mass sources with infrared excess (~25%) and X-ray emission (~6%). About 3350 heavily reddened sources with 10 < AV < 40) are most likely deeply embedded pre-main-sequence objects with an age of less than 5 × 105 yr. This group contains about 47% sources with infrared excess and 12% X-ray emitters. Cluster members later than about A0 have not yet reached the main sequence. In addition, a group of 647 protostellar candidates (1.5 < K − L < 6.9) has been detected in the cluster center as well as in the northern and southwestern bar. This population of accreting protostars argues in favor of ongoing star formation triggered by the central O stars in M17.

Molecular Outflows and a Mid-Infrared Census of the Massive Star Formation Region Associated with IRAS 18507+0121

We have observed the central region of the infrared-dark cloud filament associated with IRAS 18507+0121 at millimeter wavelengths in CO(J = 1-0), ^(13)CO(J = 1-0), and C^(18)O(J = 1-0) line emission and with Spitzer at mid-infrared wavelengths. Five massive outflows from two cloud cores were discovered. Three outflows are centered on or near an ultracompact (UC) H II region (G34.4+0.23), while the remaining two outflows originate from the millimeter core G34.4+0.23 MM. Modeling of the spectral energy distributions of the mid-infrared sources identified 31 young stellar objects in the filament with a combined stellar mass of ~127 ± 27 M_☉. An additional 22 sources were identified as probable cluster members based on the presence of strong 24 μm emission. The total star formation efficiency in the G34.4 cloud filament is estimated to be ~7%, while the massive and intermediate-mass star formation efficiency in the entire cloud filament is estimated to be roughly 2%. A comparison of the gravitational binding energy with the outflow kinetic energy suggests that the compact core containing G34.4+0.23 MM is being destroyed by its molecular outflows, whereas the outflows associated with the more massive core surrounding the G34.4 UC H II region are not likely to totally disrupt the cloud. In addition, a qualitative evaluation of the region appears to suggest that stars in this region may have formed in two stages: first lower mass stars formed and then, a few Myr later, the more massive stars began to form.

A frozen super-Earth orbiting a star at the bottom of the main sequence

Context. Microlensing is a unique method to probe low mass exoplanets beyond the snow line. However, the scientific potential of the new microlensing planet discovery is often unfulfilled due to lack of knowledge of the properties of the lens and source stars. The discovery light curve of the super Earth MOA-2007-BLG-192Lb suffers from significant degeneracies that limit what can be inferred about its physical properties. Aims. High resolution adaptive optics images allow us to solve this problem by resolving the microlensing target from all unrelated background stars, yielding the unique determination of magnified source and lens fluxes. This estimation permits the solution of our microlens model for the mass of the planet and its host and their physical projected separation. Methods. We observed the microlensing event MOA-2007-BLG-192 at high angular resolution in JHKs with the NACO adaptive optics system on the VLT while the object was still amplified by a factor 1.23 and then at baseline 18 months later. We analyzed and calibrated the NACO photometry in the standard 2MASS system in order to accurately constrain the source and the lens star fluxes. Results. We detect light from the host star of MOA-2007-BLG-192Lb, which significantly reduces the uncertainties in its characteristics as compared to earlier analyses. We find that MOA-2007-BLG-192L is most likely a very low mass late type M-dwarf (0.084 +0.015 −0.012 M� ) at a distance of 660

A deep look into the cores of young clusters - I.

Context. Nearby young clusters are privileged places to study the star formation history. Over the last decade, the σ-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function. Aims. To extend previous studies of this association to its core, we searched for ultracool members and new multiple systems within the 1. � 5 × 1. 5 central region of the cluster. Methods. We obtained deep multi-conjugate adaptive optics (MCAO) images of the core of the σ-Orionis cluster with the prototype MCAO facility MAD at the VLT using the H and Ks filters. These images allow us to detect companions fainter by ΔH ≈ 5 mag as close as 0. �� 2 on a typical source with H = 14.5 mag. These images were complemented by archival SofI K s-band images and Spitzer IRAC and MIPS mid-infrared images Results. We report the detection of 2 new visual multiple systems, one being a candidate binary proplyd and the other one a low mass companion to the massive star σ Ori E. Of the 36 sources detected in the images, 25 have a H-band luminosity lower than the expected planetary mass limit for members, and H − Ks color consistent with the latest theoretical isochrones. Nine objects have additional Spitzer photometry and spectral energy distribution consistent with them being cluster members. One of them has a spectral energy distribution from H to 3.6 μm consistent with that of a 5.5 MJup cluster member. Complementary NTT/SofI and Spitzer photometry allow us to confirm the nature and membership of two L-dwarf planetary mass candidates.

\sigma

Star clusters are born in a highly compact configuration, typically with radii of less than about 1 pc roughly independently of mass. Since the star formation efficiency is less than 50 per cent by observation and because the residual gas is removed from the embedded cluster, the cluster must expand. In the process of doing so it only retains a fraction f st of its stars. To date there are no observational constraints for f st , although N-body calculations by Kroupa, Aarseth & Hurley suggest it to be about 20-30 per cent for Orion-type clusters. Here we use the data compiled by Testi et al., Testi, Palla & Natta and Testi, Palla & Natta for clusters around young Ae/Be stars and by de Wit et al. and de Wit et al. around young O stars and the study of de Zeeuw et al. of OB associations and combine these measurements with the expected number of stars in clusters with primary Ae/Be and O stars, respectively, using the empirical correlation between maximal stellar mass and star cluster mass of Weidner & Kroupa. We find that f st < 50 per cent with a decrease to higher cluster masses/more massive primaries. The interpretation would be that cluster formation is very disruptive. It appears that clusters with a birth stellar mass in the range 10 - 10 3 M ⊙ keep at most 50 per cent of their stars.

-Orionis

While the formation of low-mass stars has become a well-studied process, it is still difficult to verify a similar evolutionary sequence for massive stars. Although several young stages from massive starless cores to massive protostellar candidates with jets and outflows have been observed, massive star/disk systems whose properties can be inferred uniquely are rare. The final stage of this sequence, i.e., a newborn massive star that is still surrounded by a remnant disk, is missing. This is probably a consequence of the rapid evolution of these systems and the early destruction of the disk in the vicinity of a massive star. We report on an optically visible young massive star (IRS 15) within M17 that displays a huge IR excess. This fortunate coincidence offers the rare opportunity to investigate the star as well as its circumstellar environment in great detail. We have performed both optical and infrared photometry and spectroscopy of the stellar source; in addition, its circumstellar environment has been investigated by mid-infrared imaging. Our data suggest that IRS 15 is a star of about 26 M☉ surrounded by a huge remnant disk of about half a Jupiter mass of dust. From this, we corroborate that massive stars can form by disk accretion and conclude that also their circumstellar disks evolve like those of low-mass stars.

On the infant weight loss of low-to intermediate-mass star clusters

Context. The frequency of brown dwarf and planetary-mass companions around intermediate-mass stars is still unknown. Imaging and radial velocity surveys have revealed a small number of substellar companions to these stars. Aims. In the course of an imaging survey we detected a visual companion to the intermediate-mass star HR 6037. We here confirm it as a co-moving substellar object. Methods. We present two epoch observations of HR 6037, an A6-type star with a companion candidate at 6. 67 and position angle of 294 degrees. We also analyze near-infrared spectroscopy of the companion. Results. Two epoch observations of HR 6037 have allowed us to confirm HR 6037 B as a co-moving companion. Its J and H band spectra suggest that the object has a spectral type of M9, with a surface gravity that is intermediate between a 10 Myr dwarf and a field dwarf of the same spectral type. The comparison of its Ks-band photometry with evolutionary tracks allows us to derive a mass, effective temperature, and surface gravity of 62 ± 20 MJup, Teff = 2330 ± 200 K, and log g = 5.1 ± 0.2, respectively. The low binary mass ratio, q ∼ 0.03, and its long orbital period, ∼5000 yr, make HR 6037 a rare and uncommon binary system.

A REMNANT DISK AROUND A YOUNG MASSIVE STAR

We investigate the morphology and the evolutionary stage of the hypercompact H II region M17-UC1 using observations at infrared wavelengths and NIR radiative transfer modeling. For the first time, this region is resolved into two emission areas separated by a dark lane reminiscent of an obscuring silhouette caused by a circumstellar disk. So far, the observational data as well as model calculations suggest that M17-UC1 is surrounded by a disk of cool dust. This direct detection of a circumstellar disk candidate around a hypercompact H II region is in agreement with the expectations of the disk accretion model for high-mass star formation.

A brown dwarf companion to the intermediate-mass star HR 6037

We present results of the first sub-arcsec resolution mid infrared survey of the southern hemisphere giant H ii region NGC 3603. We have observed selected fields in the vicinity of the OB cluster at wavelengths of 11.9  μ m and 18  μ m using TIMMI 2 mounted on the ESO 3.6 m telescope. These fields comprise areas with dense molecular cores, embedded near infrared sources as well as several OH, H 2 O and CH 3 OH maser sources, which give indications of ongoing star formation processes. We report the detection of 36 mid infrared point sources and additionally provide flux measurements for 42 knots of diffuse emission. In the area surveyed the protostar IRS 9A is found to be the most luminous source at both 11.9  μ m and 18  μ m. Located in its immediate vicinity two more sources (IRS 9B and IRS 9C) also exhibit significant 11.9  μ m and 18  μ m emission, thus providing further indications for IRS 9 being an association of protostars in its own right. Several other 11.9  μ m point sources are related to near infrared sources with strong K -band excess emission and/or to maser sources, which classifies them as young sources, too. In contrast, the second strongest 11.9  μ m source, IRS 4, appears to be in a more evolved stage. Towards the center of the OB cluster we observe mid infrared emission arising from the three Wolf-Rayet stars WR 43abc, providing evidence for dust production and/or the presence of plasma in their circum stellar envelopes. Spread all over the cluster, we detect a number of sources with mid infrared fluxes close to the sensitivity limit (~0.01 Jy) of our 11.9  μ m data, which apparently have very red