A. Bik

Clusters in the inner spiral arms of M 51: The cluster IMF and the formation history

We present the results of an analysis of the HST-WFPC2 observations of the interacting galaxy M51. From the observations in 5 broadband filters (UBVRI) and two narrowband filters (Hα and (OIII)) we study the cluster population in a region of 3.2 ×3.2 kpc 2 in the inner spiral arms of M51, at a distance of about 1 to 3 kpc from the nucleus. We found 877 cluster candidates and we derived their ages, initial masses and extinctions by means of a comparison between the observed spectral energy distribution and the predictions from cluster synthesis models for instantaneous star formation and solar metallicity. The lack of (OIII) emission in even the youngest clusters with strong Hα emission, indicates the absence of the most massive stars and suggests a mass upper limit of about 25 to 30 M� . The mass versus age distribution of the clusters shows a drastic decrease in the number of clusters with age, much more severe than can be expected on the basis of evolutionary fading of the clusters. This indicates that cluster dispersion is occurring on a timescale of 10 Myr or longer. The cluster initial mass function has been derived from clusters younger than 10 Myr by a linear regression fit of the cumulative mass distribution. This results in an exponent α = −dlogN(M)/dlog (M) = 2.1 ± 0.3 in the range of 2.5 × 10 3 2 × 10 4 M� . In the restricted range of 2.5 × 10 3 < M < 2 × 10 4 Mwe find α = 2.0 ± 0.05. This exponent is very similar to the value derived for clusters in the interacting Antennae galaxies, and to the exponent of the mass distribution of the giant molecular clouds in our Galaxy. To study the possible effects of the interaction of M51 with its companion NGC 5195 about 400 Myr ago, which triggered a huge starburst in the nucleus, we determined the cluster formation rate as a function of time for clusters with an initial mass larger than 10 4 M� . There is no evidence for a peak in the cluster formation rate at around 200 to 400 Myr ago within 2 σ accuracy, i.e. within a factor two. The formation rate of the detected clusters decreases strongly with age by about a factor 10 2 between 10 Myr and 1 Gyr. For clusters older than about 150 Myr this is due to the evolutionary fading of the clusters below the detection limit. For clusters younger than 100 Myr this is due to the dispersion of the clusters, unless one assumes that the cluster formation rate has been steadily increasing with time from 1 Gyr ago to the present time.

VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions

High-quality K-band spectra of strongly reddened point sources, deeply embedded in (ultra-)compact H II, have revealed a population of 20 young massive stars showing no photospheric absorption lines, but sometimes strong Brgamma emission. The Brgamma equivalent widths occupy a wide range (from about 1 to over 100 �; the line widths of 100-200 km s-1 indicate a circumstellar rather than a nebular origin. The K-band spectra exhibit one or more features commonly associated with massive young stellar objects (YSOs) surrounded by circumstellar material: a very red colour (J-K) ⪆ 2, CO bandhead emission, hydrogen emission lines (sometimes doubly peaked), and Fe II and/or Mg II emission lines. The large number of objects in our sample allows a more detailed definition and thorough investigation of the properties of the massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) the massive YSO candidates are located in a region delimited by the OB zero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e] supergiants. The massive YSO distribution in the CMD suggests that the majority of the objects are of similar spectral type as the Herbig Be stars, but some of them are young O stars. The spectral properties of the observed objects do not correlate with the location in the CMD. The CO emission must come from a relatively dense (~10(10) cm-3) and hot (T~2000-5000K) region, sufficiently shielded from the intense UV radiation field of the young massive star. The hydrogen emission is produced in an ionised medium exposed to UV radiation. The best geometrical solution is a dense and neutral circumstellar disk causing the CO bandhead emission, and an ionised upper layer where the hydrogen lines are produced. We present arguments that the circumstellar disk is more likely a remnant of the accretion process than the result of rapid rotation and mass loss such as in Be/B[e] stars.

Age spread in W3 main: Large Binocular Telescope/LUCI near-infrared spectroscopy of the massive stellar content

We present near-infrared multi-object spectroscopy and JHK(s) imaging of the massive stellar content of the Galactic star-forming region W3 Main, obtained with LUCI at the Large Binocular Telescope. We confirm 15 OB stars in W3 Main and derive spectral types between O5V and B4V from their absorption line spectra. Three massive young stellar objects are identified by their emission line spectra and near-infrared excess. The color-color diagram of the detected sources allows a detailed investigation of the slope of the near-infrared extinction law toward W3 Main. Analysis of the Hertzsprung-Russell diagram suggests that the Nishiyama extinction law fits the stellar population of W3 Main best (E(J -H)/ E(H - K-s) = 1.76 and RKs = 1.44). From our spectrophotometric analysis of the massive stars and the nature of their surrounding H II 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 are found for another star (IRS N1), deeply embedded in an ultracompact H II (UCH II) region, in line with the different evolutionary phases observed in the corresponding H II regions. We derive a stellar mass of W3 Main of (4 +/- 1) x 10(3) M-circle dot by extrapolating from the number of OB stars using a Kroupa initial mass function and correcting for our spectroscopic incompleteness. We have detected the photospheres of OB stars from the more evolved diffuse H II region to the much younger UCH II regions, suggesting that these stars have finished their formation and cleared away their circumstellar disks very fast. Only in the hyper-compact H II region (IRS5) do the early-type stars seem to be still surrounded by circumstellar material.

Sequential star formation in rcw 34: a spectroscopic census of the stellar content of high-mass star-forming regions

In this paper, we present VLT/SINFONI integral field spectroscopy of RCW 34 along with Spitzer/IRAC photometry of the surroundings. RCW 34 consists of three different regions. A large bubble has been detected in the IRAC images in which a cluster of intermediate- and low-mass class II objects is found. At the northern edge of this bubble, an Hii region is located, ionized by 3 OB stars, of which the most massive star has spectral type O8.5V. Intermediate-mass stars (2–3 M� ) are detected of G- and K-spectral type. These stars are still in the premain-sequence (PMS) phase. North of the Hii region, a photon-dominated region is present, marking the edge of a dense molecular cloud traced by H2 emission. Several class 0/I objects are associated with this cloud, indicating that star formation is still taking place. The distance to RCW 34 is revised to 2.5 ± 0.2 kpc and an age estimate of 2 ± 1 Myr is derived from the properties of the PMS stars inside the Hii region. Between the class II sources in the bubble and the PMS stars in the Hii region, no age difference could be detected with the present data. The presence of the class 0/I sources in the molecular cloud, however, suggests that the objects inside the molecular cloud are significantly younger. The most likely scenario for the formation of the three regions is that star formation propagated from south to north. First the bubble is formed, produced by intermediate- and low-mass stars only, after that, the Hii region is formed from a dense core at the edge of the molecular cloud, resulting in the expansion similar to a champagne flow. More recently, star formation occurred in the rest of the molecular cloud. Two different formation scenarios are possible. (1) The bubble with the cluster of low- and intermediate-mass stars triggered the formation of the O star at the edge of the molecular cloud, which in its turn induces the current star formation in the molecular cloud. (2) An external triggering is responsible for the star formation propagating from south to north.

Different evolutionary stages in the massive star-forming region S255 complex

Aims. Massive stars form in clusters, and they are often found in different evolutionary stages located close to each other. To understand evolutionary and environmental effects during the formation of high-mass stars, we observed three regions of massive star formation at different evolutionary stages, and all are found that in the same natal molecular cloud. Methods. The three regions, S255IR, S255N, and S255S, were observed at 1.3 mm with the submillimeter array (SMA), and follow-up short spacing information was obtained with the IRAM 30 m telescope. Near infrared (NIR) H + K-band spectra and continuum observations were taken for S255IR with VLT-SINFONI to study the different stellar populations in this region. Results. This combination of millimeter (mm) and near infrared data allow us to characterize different stellar populations within the young forming cluster in detail. While we find multiple mm continuum sources toward all regions, their outflow, disk, and chemical properties vary considerably. The most evolved source S255IR exhibits a collimated bipolar outflow visible in CO and H 2 emission, and the outflows from the youngest region S255S are still small and fairly confined in the regions of the mm continuum peaks. Also the chemistry toward S255IR is the most evolved, exhibiting strong emission from complex molecules, while much fewer molecular lines are detected in S255N, and in S255S we detect only CO isotopologues and SO lines. Also, rotational structures are found toward S255N and S255IR. Furthermore, a comparison of the NIR SINFONI and mm data from S255IR clearly reveal two different (proto) stellar populations with an estimated age difference of approximately 1 Myr. Conclusions. A multiwavelength spectroscopy and mapping study reveals different evolutionary phases of the star formation regions. We propose the triggered outside-in collapse star formation scenario for the bigger picture and the fragmentation scenario for S255IR.

Evidence for a hot dust-free inner disk around 51 Oph

We report on the observation of CO bandhead emission around 51 Oph (∆v = 2). A high resolving power (R � 10 000) spectrum was obtained with the infrared spectrometer ISAAC mounted on VL T − ANT U. Modeling of the profile suggests that the hot (Tgas = 2000−4000 K) and dense (nH > 10 10 cm −3 ) molecular material as probed by the CO bandhead is located in the inner AU of a Keplerian disk viewed almost edge-on. Combined with the observation of cooler gas (Tgas = 500−900 K) by ISO-SWS and the lack of cold material, our data suggest that the disk around 51 Oph is essentially warm and small. We demonstrate the presence of a dust-free inner disk that extents from the inner truncation radius until the dust sublimation radius. The disk around 51 Oph may be in a rare transition state toward a small debris disk object.

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.

Massive Binaries in High-Mass Star-forming Regions: A Multiepoch Radial Velocity Survey of Embedded O Stars

We present the first multiepoch 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 of detecting massive binaries. Our 16 targets are located in high-mass star-forming regions, and many of them are associated with known ultracompact H II 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-1 velocity differences between two epochs, proving the presence of close massive binaries. The fact that two 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-1, significantly larger than our estimated uncertainty (25 km s-1). 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.

The Lyman alpha reference sample VII. Spatially resolved H alpha kinematics

We present integral field spectroscopic observations with the Potsdam Multi-Aperture Spectrophotometer of all 14 galaxies in the z ~ 0.1 Lyman Alpha Reference Sample (LARS). We produce 2D line-of-sight velocity maps and velocity dispersion maps from the Balmer α (Hα) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies’ intrinsic Lyα radiation field. We show our kinematic maps that are spatially registered onto the Hubble Space Telescope Hα and Lyman α (Lyα) images. We can conjecture a causal connection between spatially resolved Hα kinematics and Lyα photometry for individual galaxies, however, no general trend can be established for the whole sample. Furthermore, we compute the intrinsic velocity dispersion σ0, the shearing velocity vshear, and the vshear/σ0 ratio from our kinematic maps. In general LARS galaxies are characterised by high intrinsic velocity dispersions (54 km s-1 median) and low shearing velocities (65 km s-1 median). The vshear/σ0 values range from 0.5 to 3.2 with an average of 1.5. It is noteworthy that five galaxies of the sample are dispersion-dominated systems with vshear/σ0 1. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Lyα radiation.

A VLT spectroscopic study of the ultracompact H ii region G29.96 0.02

A high quality, medium-resolution K-band spectrum has been obtained of the ultracompact H  region G29.96−0.02 with the Very Large Telescope (VLT). The slit was positioned along the symmetry axis of the cometary shaped nebula. Besides the spectrum of the embedded ionizing O star, the long-slit observation revealed the rich emission-line spectrum produced by the ionized nebula with sub-arcsec spatial resolution. The nebular spectrum includes Brγ, several helium emission lines and a molecular hydrogen line. A detailed analysis is presented of the variation in strength, velocity and width of the nebular emission lines along the slit. The results are consistent with previous observations, but the much better spatial resolution allows a critical evaluation of models explaining the cometary shape of the nebula. Our observations support neither the wind bow shock model nor the champagne flow model. The measured line ratios of the nebular hydrogen and helium lines are compared to predictions from case B recombination-line theory. The results indicate an electron temperature between 6400 and 7500 K, in good agreement with other determinations and the Galactocentric distance of 4.6 kpc. The He + /H + ratio is practically constant over the slit; we argue that He is singly ionized throughout the nebula. We review the various observational constraints on the effective temperature of the ionizing star and show that these are in agreement with its K-band spectral type of O5-O6 V.