Newly-born Stars Research Articles

Star formation in the local Universe from the CALIFA sample

The Star Formation Rate (SFR) is one of the main parameters used to analyze the evolution of galaxies through time. The need for recovering the light reprocessed by dust commonly requires the use of low spatial resolution far-infrared data. Recombination-line luminosities provide an alternative, although uncertain dust-extinction corrections based on narrow-band imaging or long-slit spectroscopy have traditionally posed a limit to their applicability. Integral Field Spectroscopy (IFS) is clearly the way to overcome such limitation. We obtain integrated H{\alpha}, ultraviolet (UV) and infrared (IR)-based SFR measurements for 272 galaxies from the CALIFA survey at 0.005 < z < 0.03 using single-band and hybrid tracers. We provide updated calibrations, both global and split by properties (including stellar mass and morphological type), referred to H{\alpha}. The extinction-corrected H{\alpha} luminosity agrees with the updated hybrid SFR estimators based on either UV or H{\alpha} plus IR luminosity over the full range of SFRs (0.03-20 M$_{\odot}$ yr$^{-1}$). The coefficient that weights the amount of energy produced by newly-born stars that is reprocessed by dust on the hybrid tracers, a$_{IR}$, shows a large dispersion. However, it does not became increasingly small at high attenuations, as expected if significant highly-obscured H$\alpha$ emission would be missed. Lenticulars, early-type spirals and type-2 AGN host galaxies show smaller coefficients due to the contribution of optical photons and AGN to dust heating. In the Local Universe the H{\alpha} luminosity derived from IFS observations can be used to measure SFR, at least in statistically-significant, optically-selected galaxy samples. The analysis of the SFR calibrations by galaxies properties could be potentially used by other works to study the impact of different selection criteria in the SFR values derived.

NGC 346: Looking in the Cradle of a Massive Star Cluster

AbstractHow does a star cluster of more than few 10,000 solar masses form? We present the case of the cluster NGC 346 in the Small Magellanic Cloud, still embedded in its natal star-forming region N66, and we propose a scenario for its formation, based on observations of the rich stellar populations in the region. Young massive clusters host a high fraction of early-type stars, indicating an extremely high star formation efficiency. The Milky Way galaxy hosts several young massive clusters that fill the gap between young low-mass open clusters and old massive globular clusters. Only a handful, though, are young enough to study their formation. Moreover, the investigation of their gaseous natal environments suffers from contamination by the Galactic disk. Young massive clusters are very abundant in distant starburst and interacting galaxies, but the distance of their hosting galaxies do not also allow a detailed analysis of their formation. The Magellanic Clouds, on the other hand, host young massive clusters in a wide range of ages with the youngest being still embedded in their giant HII regions. Hubble Space Telescope imaging of such star-forming complexes provide a stellar sampling with a high dynamic range in stellar masses, allowing the detailed study of star formation at scales typical for molecular clouds. Our cluster analysis on the distribution of newly-born stars in N66 shows that star formation in the region proceeds in a clumpy hierarchical fashion, leading to the formation of both a dominant young massive cluster, hosting about half of the observed pre–main-sequence population, and a self-similar dispersed distribution of the remaining stars. We investigate the correlation between stellar surface density (and star formation rate derived from star-counts) and molecular gas surface density (derived from dust column density) in order to unravel the physical conditions that gave birth to NGC 346. A power law fit to the data yields a steep correlation between these two parameters with a considerable scatter. The fraction of stellar over the total (gas plus young stars) mass is found to be systematically higher within the central 15 pc (where the young massive cluster is located) than outside, which suggests variations in the star formation efficiency within the same star-forming complex. This trend possibly reflects a change of star formation efficiency in N66 between clustered and non-clustered star formation. Our findings suggest that the formation of NGC 346 is the combined result of star formation regulated by turbulence and of early dynamical evolution induced by the gravitational potential of the dense interstellar medium.

Massive gaseous discs around SMBH binaries: Binary decay and tidal disruptions

We investigate the evolution of black hole binaries embedded within geometrically thin gas discs. Our results imply that such discs can produce black hole mergers for relatively low-mass binaries, and that a significant population of eccentric binaries might exist at separations of a few 0.01 pc. These binaries may be detectable due to the time-variable accretion on to the black holes. If the disc fragments, then the newly-born stars will continue driving the binary to its coalescence, although at a slower rate. Interestingly, our preliminary analysis shows that these stars will be disrupted at a rate of ∼10 −4 -2 · 10 −5 events per year per system.

The low-mass YSO CB230-A: investigating the protostar and its jet with NIR spectroscopy and Spitzer observations

Context. To investigate the earliest phases of star formation and study how newly-born stars interact with the surrounding medium, we performed a line and continuum survey at NIR and mm-wavelengths of a sample of relatively isolated Bok globules. Aims. We present a follow-up observational program of a star-forming site in the globule CB230. From narrow-band continuum observations of this site, we had discovered a bright [FeII] jet, which originates in the low-mass YSO CB230-A. We aim to investigate the physical properties of the region from where the jet is launched. Methods. Our analysis was carried out using low-resolution NIR spectra acquired with the camera NICS at the TNG telescope, with JH and HK grisms and a 1 arcsec-wide slit. These observational data were complemented with infrared photometric data from the Spitzer space telescope archive. Results. The relevant physical properties of CB230-A were constrained by SED fitting of fluxes from the NIR to the mm. The YSO spectrum exhibits a significant number of atomic and molecular emission lines and absorption features. The characteristics of this spectrum suggest that we are observing a region in the close vicinity of CB230-A, i.e. its photosphere and/or an active accretion disk. The spectra of the knots in the jet contain a large number of emission lines, including a rich set of [FeII] lines. Emission due to H2 and [FeII] are not spatially correlated, confirming that [FeII] and H2 are excited by different mechanisms, in agreement with the models where [FeII] traces dissociative J-shocks and molecular hydrogen traces slower C-shocks. By using intensity ratios involving density-sensitive [FeII] lines, we estimated the electron densities along the jet to be 6 × 10 3 –1 × 10 4 cm −3 . This indicates either high density post-shock regions of ionised gas or regions with a high degree of ionisation. Conclusions. By combining the present data with previously obtained maps at NIR- and mm-wavelengths, the emerging scenario is that CB230-A is a Class 0/I YSO driving an atomic jet that is observed to be almost monopolar probably due to its inclination to the plane of the sky and the resulting higher extinction of its red side. This primary jet appears to be sufficiently energetic to open the cavity visible in the NIR images and drive the large-scale molecular outflow observed at mm-wavelengths. CB230-A was revealed to be a good location to test the innermost structure of accreting low-mass protostars.

Star formation in molecular clouds

We examine how star formation occurs in the Galaxy and come to the following conclusions. (1) The distribution of newly-born stars in the Galaxy depends on the origin of giant-molecular-cloud complexes. For individual complexes, we favor the mechanism of Parker's instability behind galactic shocks. The production of “supercomplexes” may require the mediation of Jeans instability in the interstellar gas. (2) Magnetic fields help to support the clumps of molecular gas making up a complex against gravitational collapse. On a timescale of 107years, these fields slip by ambipolar diffusion relative to the neutral gas, leading to the formation of dense cloud cores. This timescale is the expected spread in ages of stars born in any clump. (3) When the cores undergo gravitational collapse, they usually give rise to low-mass stars on a timescale of 105years. (4) What shuts off the accretion flow and determines the mass of a new star is the onset of a powerful stellar wind. The ultimate source of energy for driving this wind in low-mass stars is the release of the energy of differential rotation acquired during the protostellar phase of evolution. The release is triggered by the entire protostar being driven convectively unstable when deuterium burning turns on.