HD 85567: A Herbig B[e] star or an interacting B[e] binary

Context. HD 85567 is an enigmatic object exhibiting the B[e] phenomenon, i.e. an infrared excess and forbidden emission lines in the optical. The object's evolutionary status is uncertain and there are conflicting claims that it is either a young stellar object or an evolved, interacting binary. Aims. To elucidate the reason for the B[e] behaviour of HD 85567, we have observed it with the VLTI and AMBER. Methods. Our observations were conducted in the K-band with moderate spectral resolution (R~1500, i.e. 200 km/s). The spectrum of HD 85567 exhibits Br gamma and CO overtone bandhead emission. The interferometric data obtained consist of spectrally dispersed visibilities, closure phases and differential phases across these spectral features and the K-band continuum. Results. The closure phase observations do not reveal evidence of asymmetry. The apparent size of HD 85567 in the K-band was determined by fitting the visibilities with a ring model. The best fitting radius, 0.8 +/- 0.3 AU, is relatively small making HD 85567 undersized in comparison to the size-luminosity relationship based on YSOs of low and intermediate luminosity. This has previously been found to be the case for luminous YSOs, and it has been proposed that this is due to the presence of an optically thick gaseous disc. We demonstrate that the differential phase observations over the CO bandhead emission are indeed consistent with the presence of a compact (~1 AU) gaseous disc interior to the dust sublimation radius. Conclusions. The observations reveal no sign of binarity. However, the data do indicate the presence of a gaseous disc interior to the dust sublimation radius. We conclude that the data are consistent with the hypothesis that HD 85567 is a YSO with an optically thick gaseous disc within a larger dust disc that is being photo-evaporated from the outer edge.Comment: Accepted for publication in A &

Spatially resolved H_2 emission from a very low-mass star

Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have been studied very little. So far, only a few CO outflows have been observed, allowing us to map the immediate circumstellar environment. We present the first spatially resolved H2 emission around IRS54 (YLW52), a ~0.1-0.2 Msun Class I source. By means of VLT SINFONI K-band observations, we probed the H2 emission down to the first ~50 AU from the source. The molecular emission shows a complex structure delineating a large outflow cavity and an asymmetric molecular jet. Thanks to the detection of several H2 transitions, we are able to estimate average values along the jet-like structure (from source position to knot D) of Av~28 mag, T~2000-3000 K, and H2 column density N(H2)~1.7x10^17 cm^-2. This allows us to estimate a mass loss rate of ~2x10^-10 Msun/yr for the warm H2 component . In addition, from the total flux of the Br Gamma line, we infer an accretion luminosity and mass accretion rate of 0.64 Lsun and ~3x10^-7 Msun/yr, respectively. The outflow structure is similar to those found in low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%), and the mass accretion rate is about one order of magnitude higher when compared to objects of roughly the same mass, pointing to the young nature of the investigated source.Comment: accepted as a Letter in A&

Infrared variability, maser activity, and accretion of massive young stellar objects

Methanol and water masers indicate young stellar objects. They often exhibit flares, and a fraction shows periodic activity. Several mechanisms might explain this behavior but the lack of concurrent infrared (IR) data complicates to identify the cause. Recently, 6.7 GHz methanol maser flares were observed, triggered by accretion bursts of high-mass YSOs which confirmed the IR-pumping of these masers. This suggests that regular IR changes might lead to maser periodicity. Hence, we scrutinized space-based IR imaging of YSOs associated with periodic methanol masers. We succeeded to extract the IR light curve from NEOWISE data for the intermediate mass YSO G107.298+5.639. Thus, for the first time a relationship between the maser and IR variability could be established. While the IR light curve shows the same period of ~34.6 days as the masers, its shape is distinct from that of the maser flares. Possible reasons for the IR periodicity are discussed.Comment: 4 pages, 3 figures, to be published in: Proceedings IAU Symposium 336 "Astrophysical Masers: Unlocking the Mysteries of the Universe", Editors: A. Tarchi, M.J. Reid & P. Castangia, updated version with hyperlinks adde

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&

Momentum-driven outflow emission from an O-type YSO: Comparing the radio jet with the molecular outflow

Aims: We want to study the physical properties of the ionized jet emission in the vicinity of an O-type young stellar object (YSO), and estimate how efficient is the transfer of energy and momentum from small- to large-scale outflows. Methods: We conducted Karl G. Jansky Very Large Array (VLA) observations, at both 22 and 45 GHz, of the compact and faint radio continuum emission in the high-mass star-forming region G023.01-00.41, with an angular resolution between 0.3" and 0.1", and a thermal rms of the order of 10 uJy/beam. Results: We discovered a collimated thermal (bremsstrahlung) jet emission, with a radio luminosity (L_rad) of 24 mJy kpc^2 at 45 GHz, in the inner 1000 AU from an O-type YSO. The radio thermal jet has an opening angle of 44 degrees and brings a momentum rate of 8 10^-3 M_sun yr^-1 km/s. By combining the new data with previous observations of the molecular outflow and water maser shocks, we can trace the outflow emission from its driving source through the molecular clump, across more than two order of magnitude in length (500 AU-0.2 pc). We find that the momentum-transfer efficiency, between the inner jet emission and the extended outflow of entrained ambient gas, is near unity. This result suggests that the large-scale flow is swept-up by the mechanical force of the radio jet emission, which originates in the inner 1000 AU from the high-mass YSO.Comment: 5 pages, 2 figures, 2 tables, accepted by Astronomy & Astrophysic

Probing the accretion-ejection connection with VLTI/AMBER: High spectral resolution observations of the Herbig Ae star HD163296

Accretion and ejection are tightly connected and represent the fundamental mechanisms regulating star formation. However, the exact physical processes involved are not yet fully understood. We present high angular and spectral resolution observations of the Br Gamma emitting region in the Herbig Ae star HD163296 (MWC275) in order to probe the origin of this line and constrain the physical processes taking place at sub-AU scales in the circumstellar region. By means of VLTI-AMBER observations at high spectral resolution (R~12000), we studied interferometric visibilities, wavelength-differential phases, and closure phases across the Br Gamma line of HD163296. To constrain the physical origin of the Br Gamma line in Herbig Ae stars, all the interferometric observables were compared with the predictions of a line radiative transfer disc wind model. The measured visibilities clearly increase within the Br Gamma line, indicating that the Br Gamma emitting region is more compact than the continuum. By fitting a geometric Gaussian model to the continuum-corrected Br Gamma visibilities, we derived a compact radius of the Br Gamma emitting region of ~0.07+/-0.02AU (Gaussian half width at half maximum; or a ring-fit radius of ~0.08+/-0.02AU). To interpret the observations, we developed a magneto-centrifugally driven disc wind model. Our best disc wind model is able to reproduce, within the errors, all the interferometric observables and it predicts a launching region with an outer radius of ~0.04AU. However, the intensity distribution of the entire disc wind emitting region extends up to ~0.16AU. Our observations, along with a detailed modelling of the Br Gamma emitting region, suggest that most of the Br Gamma emission in HD163296 originates from a disc wind with a launching region that is over five times more compact than previous estimates of the continuum dust rim radius.Comment: Accepted for publication in A&