IR diagnostics of embedded jets: velocity resolved observations of the HH34 and HH1 jets

We present VLT-ISAAC medium resolution spectroscopy of the HH34 and HH1 jets. Our aim is to derive the kinematics and the physical parameters and to study how they vary with jet velocity. We use several important diagnostic lines such as [FeII] 1.644um, 1.600um and H2 2.122um. In the inner jet region of HH34 we find that both the atomic and molecular gas present two components at high and low velocity. The [FeII] LVC in HH34 is detected up to large distances from the source (>1000 AU), at variance with TTauri jets. In H2 2.122um, the LVC and HVC are spatially separated. We detect, for the first time, the fainter red-shifted counterpart down to the central source. In HH1, we trace the jet down to ~1" from the VLA1 driving source: the kinematics of this inner region is again characterised by the presence of two velocity components, one blue-shifted and one red-shifted with respect to the source LSR velocity. In the inner HH34 jet region, ne increases with decreasing velocity. Up to ~10" from the driving source, and along the whole HH1 jet an opposite behaviour is observed instead, with ne increasing with velocity. In both jets the mass flux is carried mainly by the high-velocity gas. A comparison between the position velocity diagrams and derived electron densities with models for MHD jet launching mechanisms has been performed for HH34. While the kinematical characteristics of the line emission at the jet base can be, at least qualitatively, reproduced by both X-winds and disc-wind models, none of these models can explain the extent of the LVC and the dependence of electron density with velocity that we observe. It is possible that the LVC in HH34 represents gas not directly ejected in the jet but instead denser ambient gas entrained by the high velocity collimated jet.Comment: A&A accepte

POISSON project - III - Investigating the evolution of the mass accretion rate

As part of the POISSON project (Protostellar Optical-Infrared Spectral Survey on NTT), we present the results of the analysis of low-resolution NIR spectra 0.9-2.4 um) of two samples of YSOs in Lupus and Serpens (52 and 17 objects), with masses 0.1-2.0 Msun and ages from 10^5 to a few 10^7 yr. After determining the accretion parameters of the Lup and Ser targets by analysing their HI near-IR emission features, we added the results to those from previous regions (investigated in POISSON with the same methodology). We obtained a final catalogue (143 objects) of mass accretion rates (Macc) derived in a homogeneous fashion and analysed how Macc correlates with M* and how it evolves in time. We derived the accretion luminosity (Lacc) and Macc for Lup and Ser objects from the Br_gamma line by using relevant empirical relationships from the literature that connect HI line luminosity and Lacc. To minimise the biases and also for self-consistency, we re-derived mass and age for each source using the same set of evolutionary tracks. We observe a correlation MaccM*^2.2, similarly to what has previously been observed in several star-forming clouds. The time variation of Macc is roughly consistent with the expected evolution in viscous disks, with an asymptotic decay that behaves as t^-1.6. However, Macc values are characterised by a large scatter at similar ages and are on average higher than the predictions of viscous models. Although part of the scattering may be related to the employed empirical relationship and to uncertainties on the single measurements, the general distribution and decay trend of the Macc points are real. These findings might be indicative of a large variation in the initial mass of the disks, of fairly different viscous laws among disks, of varying accretion regimes, and of other mechanisms that add to the dissipation of the disks, such as photo-evaporation.Comment: 18 pages, 10 figures, accepted by A&

The circumstellar environment of HD50138 revealed by VLTI/AMBER at high angular resolution

HD50138 is a Herbig B[e] star with a circumstellar disc detected at IR and mm wavelength. Its brightness makes it a good candidate for NIR interferometry observations. We aim to resolve, spatially and spectrally, the continuum and hydrogen emission lines in the 2.12-2.47 micron region, to shed light on the immediate circumstellar environment of the star. VLTI/AMBER K-band observations provide spectra, visibilities, differential phases, and closure phases along three long baselines for the continuum, and HI emission in Br$\gamma$ and five high-n Pfund lines. By computing the pure-line visibilities, we derive the angular size of the different line-emitting regions. A simple LTE model was created to constrain the physical conditions of HI emitting region. The continuum region cannot be reproduced by a geometrical 2D elongated Gaussian fitting model. We estimate the size of the region to be 1 au. We find the Br$\gamma$ and Pfund lines come from a more compact region of size 0.4 au. The Br$\gamma$ line exhibits an S-shaped differential phase, indicative of rotation. The continuum and Br$\gamma$ line closure phase show offsets of $\sim$-25$\pm$5 $^o$ and 20$\pm$10$^o$, respectively. This is evidence of an asymmetry in their origin, but with opposing directions. We find that we cannot converge on constraints for the HI physical parameters without a more detailed model. Our analysis reveals that HD50138 hosts a complex circumstellar environment. Its continuum emission cannot be reproduced by a simple disc brightness distribution. Similarly, several components must be evoked to reproduce the interferometric observables within the Br$\gamma$, line. Combining the spectroscopic and interferometric data of the Br$\gamma$ and Pfund lines favours an origin in a wind region with a large opening angle. Finally, our results point to an evolved source.Comment: accepted for publication in A&

POISSON project - I - Emission lines as accretion tracers in young stellar objects: results from observations of Chamaeleon I and II sources

We present the results of the analysis of LR optical-NIR spectra (0.6-2.4 um) of a sample 47 YSOs in the ChaI and II star-forming clouds. These data are part of the POISSON project (Protostellar Optical-Infrared Spectral Survey on NTT). The aim is to determine the accretion luminosity (Lacc) and mass accretion rate (Macc) of the sources through the analysis of the detected emission features. We also aim at verifying the reliability and consistency of the existing empirical relationships connecting emission line luminosity and Lacc. We employ five tracers (OI-6300A, Ha, CaII-8542A, Pab, and Brg) to derive the accretion luminosity. The tracers provide Lacc values showing different scatters when plotted as a function of L*. The Brg seems to be the most reliable, because it gives the minimum Lacc dispersion over the entire range of L*, whereas the other tracers provide much more scattered Lacc values, which are not expected for our homogeneous sample. The comparison between Lacc(Brg) and Lacc obtained from the other tracers also shows systematic differences among the empirical relationships. These may probably be ascribed to different excitation mechanisms contributing to the line emission, which may vary between our sample and those where the relationships were calibrated. Adopting the Lacc derived from Brg, we find Lacc=0.1L*-1L* for all sources, and Macc of the order of 10^-7-10^-9 Msun/yr. The Macc derived in ChaI are proportional to M*^2, as found in other low-mass star-forming regions. The discrepancies observed in the case of Lacc(Brg) and Lacc(Pab) can be related to different intrinsic Pab/Brg, ratios. The derived ratios show 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 (10^21-10^22 cm^-3) with gas at low temperatures (<4000K).Comment: 22 pages, 8 figures, accepted for publication in A&A; institute affiliations and typos correcte

Exploring the dimming event of RW Aur A through multi-epoch VLT/X-Shooter spectroscopy

RW Aur A is a CTTS that has suddenly undergone three major dimming events since 2010. We aim to understand the dimming properties, examine accretion variability, and derive the physical properties of the inner disc traced by the CO ro-vibrational emission at NIR wavelengths (2.3 mic). We compared two epochs of X-Shooter observations, during and after the dimming. We modelled the rarely detected CO bandhead emission in both epochs to examine whether the inner disc properties had changed. The SED was used to derive the extinction properties of the dimmed spectrum and compare the infrared excess between the two epochs. Lines tracing accretion were used to derive the mass accretion rate in both states. The CO originates from a region with physical properties of T=3000 K, N$_{CO}$=1x10$^{21}$ cm$^{-2}$ and vsini=113 km/s. The extinction properties of the dimming layer were derived with the effective optical depth ranging from teff 2.5-1.5 from the UV to the NIR. The inferred mass accretion rate Macc is $1.5x 10^{-8}$ Msun/yr and $\sim 2x 10^{-8}$ Msun/yr after and during the dimming respectively. By fitting the SED, additional emission is observed in the IR during the dimming event from dust grains with temperatures of 500-700K. The physical conditions traced by the CO are similar for both epochs, indicating that the inner gaseous disc properties do not change during the dimming events. The extinction curve is flatter than that of the ISM, and large grains of a few hundred microns are thus required. When we correct for the observed extinction, Macc is constant in the two epochs, suggesting that the accretion is stable and therefore does not cause the dimming. The additional hot emission in the NIR is located at about 0.5 au from the star. The dimming events could be due to a dust-laden wind, a severe puffing-up of the inner rim, or a perturbation caused by the recent star-disc encounter.Comment: Accepted by Astronomy & Astrophysic

On the MBM12 Young Association

I present a comprehensive study of the MBM12 young association (MBM12A). By combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS) survey with new optical imaging and spectroscopy, I have performed a census of the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X 1.4deg field encompassing the MBM12 cloud. I find five new members with masses of 0.1-0.4 Msun and a few additional candidates that have not been observed spectroscopically. From an analysis of optical and IR photometry for stars in the direction of MBM12, I identify M dwarfs in the foreground and background of the cloud. By comparing the magnitudes of these stars to those of local field dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud; it is not the nearest molecular cloud and is not inside the local bubble of hot ionized gas as had been implied by previous distance estimates of 50-100 pc. I have also used Li strengths and H-R diagrams to constrain the absolute and relative ages of MBM12A and other young populations; these data indicate ages of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations. MBM12A may be a slightly evolved version of the aggregates of young stars within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2 Myr).Comment: to be published in The Astrophysical Journal, 41 pages, 14 figures, also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm