The Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) carried out a 250, 350, and 500 Delta *mm survey of the galactic plane encompassing the Vela Molecular Ridge, with the primary goal of ...identifying the coldest dense cores possibly associated with the earliest stages of star formation. Here, we present the results from observations of the Vela-D region, covering about 4 deg2, in which we find 141 BLAST cores. We exploit existing data taken with the Spitzer MIPS, IRAC, and SEST-SIMBA instruments to constrain their (single-temperature) spectral energy distributions, assuming a dust emissivity index Delta *b = 2.0. This combination of data allows us to determine the temperature, luminosity, and mass of each BLAST core, and also enables us to separate starless from protostellar sources. We also analyze the effects that the uncertainties on the derived physical parameters of the individual sources have on the overall physical properties of starless and protostellar cores, and we find that there appear to be a smooth transition from the pre- to the protostellar phase. In particular, for protostellar cores we find a correlation between the MIPS24 flux, associated with the central protostar, and the temperature of the dust envelope. We also find that the core mass function of the Vela-D cores has a slope consistent with other similar (sub)millimeter surveys.
The Vela-D region, according to the nomenclature given by Murphy & May, of the star-forming complex known as the Vela molecular ridge (VMR), has recently been analyzed in detail by Olmi, who studied ...the physical properties of 141 pre- and proto-stellar cold dust cores, detected by the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) during a much larger (55 deg2) Galactic plane survey encompassing the whole VMR. This survey's primary goal was to identify the coldest dense dust cores possibly associated with the earliest phases of star formation. In this work, the dynamical state of the Vela-D cores is analyzed. Comparison to dynamical masses of a sub-sample of the Vela-D cores estimated from the 13CO survey of Elia is complicated by the fact that the 13CO linewidths are likely to trace the lower density intercore material, in addition to the dense gas associated with the compact cores observed by BLAST. In fact, the total internal pressure of these cores, if estimated using the 13CO linewidths, appears to be higher than the cloud ambient pressure. If this were the case, then self-gravity and surface pressure would be insufficient to bind these cores and an additional source of external confinement (e.g., magnetic field pressure) would be required. However, if one attempts to scale down the 13CO linewidths, according to the observations of high-density tracers in a small sample of sources, then most proto-stellar cores would be effectively gravitationally bound.
Context. We present the results of the analysis of low-resolution optical-near IR spectroscopy (0.6-2.4 mum) of a sample (47 sources) of Class I and Class II young stellar objects in the Chamaeleon I ...and II star-forming clouds. These data are part of the POISSON project (Protostellar Optical-Infrared Spectral Survey On NTT). Aims: The aim of the observations is to determine the accretion luminosity (Lacc) and mass accretion rate (Macc) of the sources through the analysis of the detected emission features. Taking advantage of the wide wavelength range covered by our spectra, we also aim at verifying the reliability and consistency of the existing empirical relationships connecting emission line luminosity and Lacc. Methods: We employ five different tracers (O i lambda6300, Halpha, Ca ii lambda8542, Pabeta, and Brgamma) to derive the accretion luminosity, and critically discuss the various determinations in the light of the source properties. Results: The tracers provide Lacc values characterised by different scatters when plotted as a function of L*. The Brgamma relation appears to be the most reliable, because it gives the minimum dispersion of Lacc over the entire range of L*, whereas the other tracers, in particular Halpha, provide much more scattered Lacc results, which are not expected for the homogeneous sample of targets we are observing. The direct comparison between Lacc(Brgamma) and the accretion luminosity obtained from the other four tracers also shows systematic differences in the results provided by the empirical relationships. These may probably be ascribed to different excitation mechanisms that contribute to the line emission, which may vary between our sample and those where the relationships have been calibrated, which were mostly based on observations in Taurus. Adopting the accretion luminosities estimates derived from the Brgamma line, we infer Lacc in the range 0.1 L*-1 L* for all sources, and Macc of the order 10-7-10-9 M yr-1, in the range of values commonly obtained for Class II objects. The mass accretion rates derived in Cha I are roughly proportional to M*2, in agreement with the results found in other low-mass star-forming regions. We find that the discrepancies observed in the case of Lacc(Brgamma) and Lacc(Pabeta) can be related to different intrinsic Pabeta/Brgamma ratios. The derived ratios point to the existence of two different emission modalities, one that agrees with predictions of both wind and accretion models, the other suggesting optically thick emission from relatively small regions (1021-1022 cm2) with gas at low temperatures (
Observations of pre-main sequence objects (T Tauri, Herbig Ae/Be and FU Orionis stars) obtained with the instrumentation on board the Infrared Space Observatory (ISO) are reviewed. All the ...observations have been mainly carried out by using the two spectrographs SWS and LWS, adopting their low resolution modes and such data have been used both for lines detection and to reconstruct the spectral energy distributions. Line emission and photometric behaviour of pre-main sequence objects have been analyzed in the framework of the current models, discussing the agreement (or disagreement) with them and trying to derive the questions which should be answered by the forthcoming FIR instrumentation.
This paper presents the observations of Cloud D in the Vela Molecular Ridge, obtained with the Infrared Array Camera (IRAC) camera on board the Spitzer Space Telescope at the wavelengths lambda = ...3.6, 4.5, 5.8, and 8.0 mum. A photometric catalog of point sources, covering a field of approximately 1.2 deg2, has been extracted and complemented with additional available observational data in the millimeter region. Previous observations of the same region, obtained with the Spitzer MIPS camera in the photometric bands at 24 mum and 70 mum, have also been reconsidered to allow an estimate of the spectral slope of the sources in a wider spectral range. A total of 170,299 point sources, detected at the 5sigma sensitivity level in at least one of the IRAC bands, have been reported in the catalog. There were 8796 sources for which good quality photometry was obtained in all four IRAC bands. For this sample, a preliminary characterization of the young stellar population based on the determination of spectral slope is discussed; combining this with diagnostics in the color-magnitude and color-color diagrams, the relative population of young stellar objects (YSOs) in different evolutionary classes has been estimated and a total of 637 candidate YSOs have been selected. The main differences in their relative abundances have been highlighted and a brief account for their spatial distribution is given. The star formation rate has also been estimated and compared with the values derived for other star-forming regions. Finally, an analysis of the spatial distribution of the sources by means of the two-point correlation function shows that the younger population, constituted by the Class I and flat-spectrum sources, is significantly more clustered than the Class II and III sources.
We present a study of the infrared variability of young stellar objects by means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained in observations separated in time by about ...six months. By using the same space-born IR instrumentation, this study eliminates all the unwanted effects due to differences in sensitivity, confusion, saturation, calibration, and filter bandpasses, issues that are usually unavoidable when comparing catalogs obtained from different instruments. The VMR-D map covers about 1.5 deg2 of a site where star formation is actively ongoing. We are interested in accreting pre-main sequence variables whose luminosity variations are due to intermittent events of disk accretion (i.e., active T Tauri stars and EXor-type objects). The variable objects have been selected from a catalog of more than 170,000 sources detected at an S/N >= 5. We then searched the sample of variables for ones whose photometric properties such as IR excess, color-magnitude relationships, and spectral energy distribution, are as close as possible to those of known EXor's. Indeed, the latter are monitored in a more systematic way than T Tauri stars and the mechanisms that regulate the observed phenomenology are exactly the same. Hence, the modalities of the EXor behavior are adopted as driving criterion for selecting variables in general. We ultimately selected 19 bona fide candidates that constitute a well defined sample of new variable targets for further investigation (monitoring, spectroscopy). Out of these, 10 sources present a Spitzer MIPS 24 Delta *mm counterpart, and have been classified as three Class I, five flat spectrum, and two Class II objects, while the spectral energy distribution of the other nine sources is compatible with evolutionary phases older than Class I. This is consistent with what is known about the small sample of known EXor's, whose properties have driven the present selection and suggests that the accretion flaring or EXor stage might come as a ClassI/ClassII transition. We also present new prescriptions that can be useful in future searches for accretion variables in large IR databases.
The EXor phenomenon Lorenzetti, Dario
arXiv.org,
03/2016
Paper, Journal Article
Odprti dostop
Recent results obtained for the eruptive variables (EXors) are reviewed. These data span from X-rays to the sub-mm band and are presented along with perspectives for future observations achievable ...with the new advanced instrumentation.
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