arXiv subjects

Arjan Bik

Publications and source records attributed to Arjan Bik.

58 records · Page 4Linked to original sources

arXiv2011

Dissecting high-mass star-forming regions; tracing back their complex formation history

We present near-infrared JHKs imaging as well as K-band multi-object spectroscopy of the massive stellar content of W3 Main using LUCI at the LBT. We confirm 13 OB stars by their absorption line spectra in W3 Main and spectral types between O5V and B4V have been found. Three massive Young Stellar Objects are identified by their emission line spectra and near-infrared excess. From our spectrophotometric analysis of the massive stars and the nature of their surrounding HII regions we derive the evolutionary sequence of W3 Main and we find evidence of an age spread of at least 2-3 Myr. While the most massive star (IRS2) is already evolved, indications for high-mass pre--main-sequence evolution is found for another star (IRS N1), deeply embedded in an ultra compact HII region, in line with the different evolutionary phases observed in the corresponding HII regions. We have detected the photospheres of OB stars from the more evolved diffuse HII region to the much younger UCHII regions, suggesting that the OB stars have finished their formation and cleared away their possible circumstellar disks very fast. Only in the hyper-compact HII region (IRS5), the early type stars are still surrounded by circumstellar material.

astro-ph.GA↗

arXiv2011

The Intermediate-mass Young Stellar Object 08576nr292: Discovery of a disk-jet system

We present observations of the embedded massive young stellar object (YSO) candidate 08576nr292, obtained with X-shooter and SINFONI on the ESO Very Large Telescope (VLT). The flux-calibrated, medium-resolution X-shooter spectrum (300 - 2500 nm) includes over 300 emission lines, but no (photospheric) absorption lines and is consistent with a reddened disk spectrum. Among the emission lines are three hydrogen series and helium lines, both permitted and forbidden metal lines, and CO first-overtone emission. A representative sample of lines with different morphologies is presented. The H alpha and Ca II triplet lines are very strong, with profiles indicative of outflow and - possibly - infall, usually observed in accreting stars. These lines include a blue-shifted absorption component at ~ -125 km/s. The He I and metal-line profiles are double-peaked, with a likely origin in a circumstellar disk. The forbidden lines, associated with outflow, have a single blue-shifted emission component centered at -125 km/s, coinciding with the absorption components in H alpha and Ca II. SINFONI H- and K-band integral-field spectroscopy of the cluster environment demonstrates that the [Fe II] emission is produced by a jet originating at the location of 08576nr292. Because the spectral type of the central object cannot be determined, its mass remains uncertain. We argue that 08576nr292 is an intermediate-mass YSO with a high accretion rate Mdot_acc ~ 10^-5 - 10^-6 Msun/yr. These observations demonstrate the potential of X-shooter and SINFONI to study in great detail an accretion disk-jet system, rarely seen around the more massive YSOs.

astro-ph.SR↗

arXiv2006

Massive binaries in high-mass star-forming regions: A multi-epoch radial velocity survey of embedded O-stars

We present the first multi-epoch radial velocity study of embedded young massive stars using near-infrared spectra obtained with ISAAC mounted at the ESO Very Large Telescope, with the aim to detect massive binaries. Our 16 targets are located in high-mass star-forming regions and many of them are associated to known ultracompact HI I regions, whose young age ensures that dynamic evolution of the clusters did not influence the intrinsic binarity rate. We identify two stars with about 90 km/s velocity differences between two epochs proving the presence of close massive binaries. The fact that 2 out of the 16 observed stars are binary systems suggests that at least 20% of the young massive stars are formed in close multiple systems, but may also be consistent with most if not all young massive stars being binaries. In addition, we show that the radial velocity dispersion of the full sample is about 35 km/s, significantly larger than our estimated uncertainty (25 km/s). This finding is consistent with similar measurements of the young massive cluster 30 Dor which might have a high intrinsic binary rate. Furthermore, we argue that virial cluster masses derived from the radial velocity dispersion of young massive stars may intrinsically overestimate the cluster mass due to the presence of binaries.

astro-ph↗

arXiv2005

Massive Young Stellar Objects in high-mass star-forming regions

High-quality K-band spectra of point sources, deeply embedded in massive star-forming regions, have revealed a population of 20 young massive stars showing no photospheric absorption lines, but only emission lines. The K-band spectra exhibit one or more features commonly associated with massive Young Stellar Objects surrounded by circumstellar material: a very red color (J-K) = 2, CO bandhead emission, hydrogen emission lines (sometimes doubly peaked), and FeII and/or MgII emission lines. The CO emission comes from a relatively dense (~10^10 cm^(-3)) and hot (T ~ 2000-5000 K) region, sufficiently shielded from the intense UV radiation field of the young massive star. Modeling of the CO-first overtone emission shows that the CO gas is located within 5 AU of the star. The hydrogen emission is produced in an ionized medium exposed to UV radiation. The best geometrical configuration is a dense and neutral circumstellar disk causing the CO bandhead emission, and an ionized upper layer where the hydrogen lines are produced. We argue that the circumstellar disk is likely a remnant of the accretion via a circumstellar disk.

astro-ph↗