The IMF and Star Formation History of the Stellar Clusters in the Vela D Cloud

We present the results of a Near-Infrared deep photometric survey of a sample of six embedded star clusters in the Vela-D molecular cloud, all associated with luminous (~10^3 Lsun) IRAS sources. The clusters are unlikely to be older than a few 10^6 yrs, since all are still associated with molecular gas. We employed the fact that all clusters lie at the same distance and were observed with the same instrumental setting to derive their properties in a consistent way, being affected by the same instrumental and observational biases. We extracted the clusters' K Luminosity Functions (KLF) and developed a simple method to correct them for extinction, based on colour-magnitude diagrams. The reliability of the method has been tested by constructing synthetic clusters from theoretical tracks for pre-main sequence stars and a standard Initial Mass Function (IMF). The clusters' IMFs have been derived from the dereddened KLFs by adopting a set of pre-main sequence evolutionary tracks and assuming coeval star formation. All clusters are small (~100 members) and compact (radius \~0.1-0.2 pc); their most massive stars are intermediate-mass (~2-10 Msun) ones. The dereddened KLFs are likely to arise from the same distribution, suggesting that the selected clusters have quite similar IMFs and star formation histories. The IMFs are consistent with those derived for field stars and clusters. Adding them together we found that the ``global'' IMF appears steeper at the high-mass end and exhibits a drop-off at ~10 Msun. In fact, a standard IMF would predict a star with M>22.5 Msun within one of the clusters, which is not found. Hence, either high-mass stars need larger clusters to be formed, or the IMF of the single clusters is steeper at the high-mass end because of the physical conditions in the parental gas.Comment: 17 pages, 14 figures, to be published in Astronomy & Astrophysic

Looking for outflow and infall signatures in high mass star forming regions

(Context) Many physical parameters change with time in star forming regions. Here we attempt to correlate changes in infall and outflow motions in high mass star forming regions with evolutionary stage using JCMT observations. (Aims) From a sample of 45 high mass star forming regions in three phases of evolution, we investigate the presence of established infall and outflow tracers to determine whether there are any trends attributable to the age of the source. (Methods) We obtained JCMT observations of HCO+/H13CO+ J=4-3 to trace large scale infall, and SiO J=8-7 to trace recent outflow activity. We compare the infall and outflow detections to the evolutionary stage of the host source (high mass protostellar objects, hypercompact HII regions and ultracompact HII regions). We also note that the integrated intensity of SiO varies with the full width at half maximum of the H13CO+. (Results) We find a surprising lack of SiO detections in the middle stage (Hypercompact HII regions), which may be due to an observational bias. When SiO is detected, we find that the integrated intensity of the line increases with evolutionary stage. We also note that all of the sources with infall signatures onto Ultracompact HII regions have corresponding outflow signatures as well.Comment: 9 pages, 5 figures, 5 tables. Accepted by A&

CARMA interferometric observations of 2MASS J044427+2512: the first spatially resolved observations of thermal emission of a brown dwarf disk

We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular resolution of the CARMA observations (0.16 arcsec) allows us to spatially resolve for the first time the thermal emission from dust around a brown dwarf. We analyze the interferometric visibilities and constrain the disk outer radius adopting disk models with power-law radial profiles of the dust surface density. In the case of a power-law index equal to or lower than 1, we obtain a disk radius in the range of about 15 - 30 AU, while larger disks are inferred for steeper radial profiles. By combining this information on the disk spatial extent with the sub-mm spectral index of this source we find conclusive evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss the implications of our results on the models of dust evolution in proto-planetary disks and brown dwarf formation.Comment: 14 pages, 3 figures, Accepted for publication in ApJ Letter

Diamonds in HD 97048

We present adaptive optics high angular resolution (\sim0\farcs1) spectroscopic observations in the 3 $\mu$m region of the Herbig Ae/Be star HD 97048. For the first time, we spatially resolve the emission in the diamond features at 3.43 and 3.53 $\mu$m and in the adjacent continuum. Using both the intensity profiles along the slit and reconstructed two-dimensional images of the object, we derive full-width at half-maximum sizes consistent with the predictions for a circumstellar disk seen pole-on. The diamond emission originates in the inner region ($R \lesssim 15$ AU) of the disk.Comment: ApJLetter, in pres

Dust-to-gas ratio resurgence in circumstellar disks due to the formation of giant planets: the case of HD 163296

The amount of dust present in circumstellar disks is expected to steadily decrease with age due to the growth from micron-sized particles to planetesimals and planets. Mature circumstellar disks, however, can be observed to contain significant amounts of dust and possess high dust-to-gas ratios. Using HD 163296 as our case study, we explore how the formation of giant planets in disks can create the conditions for collisionally rejuvenating the dust population, halting or reversing the expected trend. We combine N-body simulations with statistical methods and impact scaling laws to estimate the dynamical and collisional excitation of the planetesimals due to the formation of HD 163296's giant planets. We show that this process creates a violent collisional environment across the disk that can inject collisionally produced second-generation dust into it, significantly contributing to the observed dust-to-gas ratio. The spatial distribution of the dust production can explain the observed local enrichments in HD 163296's inner regions. The results obtained for HD 163296 can be extended to any disk with embedded forming giant planets and may indicate a common evolutionary stage in the life of such circumstellar disks. Furthermore, the dynamical excitation of the planetesimals could result in the release of transient, non-equilibrium gas species like H2O, CO2, NH3 and CO in the disk due to ice sublimation during impacts and, due to the excited planetesimals being supersonic with respect to the gas, could produce bow shocks in the latter that could heat it and cause a broadening of its emission lines.Comment: 18 pages, 9 figures, 2 tables. Accepted for publication on The Astrophysical Journa

X-Shooter study of accretion in ρ\rho-Ophiucus: very low-mass stars and brown dwarfs

We present new VLT/X-Shooter optical and NIR spectra of a sample of 17 candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived SpT and Av for all the targets, and then we determined their physical parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or close to the hydrogen-burning limit. Using the intensity of various emission lines present in their spectra, we determined the Lacc and Macc for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This leads us to conclude that we do not find evidence for a different dependence of Macc with M* when comparing low-mass stars and BDs. Moreover, we find a similar small (1 dex) scatter in the Macc-M* relation as in some of our recent works in other star-forming regions, and no significant differences in Macc due to different ages or properties of the regions. The latter result suffers, however, from low statistics and sample selection biases in the current studies. The small scatter in the Macc-M* correlation confirms that Macc in the literature based on uncertain photospheric parameters and single accretion indicators, such as the Ha width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the photospheric and accretion properties allows us to properly study the evolution of disk accretion rates.Comment: accepted for publication in Astronomy & Astrophysics. Abstract shortened to fit arXiv constraint

The effect of local optically thick regions in the long-wave emission of young circumstellar disks

Multi-wavelength observations of protoplanetary disks in the sub-millimeter continuum have measured spectral indices values which are significantly lower than what is found in the diffuse interstellar medium. Under the assumption that mm-wave emission of disks is mostly optically thin, these data have been generally interpreted as evidence for the presence of mm/cm-sized pebbles in the disk outer regions. In this work we investigate the effect of possible local optically thick regions on the mm-wave emission of protoplanetary disks without mm/cm-sized grains. A significant local increase of the optical depth in the disk can be caused by the concentration of solid particles, as predicted to result from a variety of proposed physical mechanisms. We calculate the filling factors and implied overdensities these optically thick regions would need to significantly affect the millimeter fluxes of disks, and we discuss their plausibility. We find that optically thick regions characterized by relatively small filling factors can reproduce the mm-data of young disks without requesting emission from mm/cm-sized pebbles. However, these optically thick regions require dust overdensities much larger than what predicted by any of the physical processes proposed in the literature to drive the concentration of solids. We find that only for the most massive disks it is possible and plausible to imagine that the presence of optically thick regions in the disk is responsible for the low measured values of the mm spectral index. For the majority of the disk population, optically thin emission from a population of large mm-sized grains remains the most plausible explanation. The results of this analysis further strengthen the scenario for which the measured low spectral indices of protoplanetary disks at mm wavelengths are due to the presence of large mm/cm-sized pebbles in the disk outer regions.Comment: 13 pages, 2 figures, A&A in pres