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

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 &

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

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&

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&

SIALIDASE NEU3 EXPRESSION IN A HUMAN MODEL OF CARDIAC ISCHEMIA AND ITS INTERPLAY WITH THE HYPOXIA-INDUCIBLE FACTOR (HIF-1) SIGNALING PATHWAY.

The hypoxic condition determines several functional consequences that ultimately lead to cellular death and irreversible damage to cardiac myocytes. Under hypoxic condition, cells activate several protective pathways; among them, HIF-1\u3b1 plays a key role in controlling cellular response to hypoxia at molecular level. However, HIF-1\u3b1 regulatory mechanisms are extremely complex. On these premises, the present work was based on the hypothesis that NEU3 sialidase, a glycolytic enzyme ubiquitously expressed over the plasmatic membrane, can have a regulatory activity on HIF-1\u3b1 expression in hypoxic/ischemic cardiac myocytes. The experiments performed in this in-vitro model allowed us to draw the following conclusions: 1) Endogenous NEU3 sialidase expression and activity are up-regulated in murine skeletal muscle cells (C2C12) upon oxygen starvation, leading to a signaling cascade resulting in the activation of HIF-1\u3b1. 2) Moreover, induced overexpression of NEU3 significantly increases HIF-1\u3b1 expression and cell resistance to hypoxic stress, whereas NEU3 silencing causes the opposite effects and renders myoblasts more susceptible to apoptosis. 3) The hypoxia-driven activation of NEU3 sialidase can activate the EGFR prosurvival signaling pathway by controlling the content of ganglioside GM3. Furthermore, we demonstrated that NEU3 overexpression causes a reduction of ganglioside GM3, which is known to block EGFR autophosphorylation. Then resulted were extended from skeletal muscle to cardiac myocytes, particularly aiming to ascertain the role of NEU3 in activating the human cardiomyocyte response to hypoxia. Particularly, we evaluated if NEU3 activation occurred in human cardiomyocytes using two different models: 1) A model of acute cardiac ischemia achieved during aortic cross-clamp time and extracorporeal circulation in adult patient submitted to cardiac surgical procedures. 2) A model of chronic hypoxia in neonates and young patients operated for cyanogen congenital cardiac defects. In the acute model of cardiac ischemia, we harvested a sample of right atrial appendage just before and after aortic cross-clamping, during routine adult cardiac surgery procedure. However, no significant activation of NEU3 and HIF-1\u3b1 was evident in cardiac sample harvested before and after aortic cross-clamping. In our opinion there are several possible explanations for the lack of NEU3 and HIF-1\u3b1 increased expression in the cardiac surgery model. First, it is possible that in the in-vivo setting the mean aortic cross-clamp time was too short (mean time = 79 minutes) to elucidate the same response that we observed in the in vitro model, where the cells were incubated under hypoxic conditions for at least 12 hours. Secondly, and most important in our opinion, the technique of myocardial protection, especially cardioplegic arrest and hypothermia, by protecting the myocardium from the ischemic injury could have limited NEU3 and HIF-1\u3b1 expression in our samples. To overcome these limitation, in the final part of my PhD program we evaluated HIF-1\u3b1 and NEU3 expression in a human in-vivo model of chronic cardiac hypoxia, studying patients affected by cyanotic cardiac defects submitted to surgical correction. In this model of chronic hypoxia, we observed a significant increase in NEU3 expression and activity in cyanotic patients. Furthermore, a significant increase of EGFR was observed, supporting the hypothesis that this signaling pathway is upregulated by the sialidase NEU3. Indeed we observed an increase in expression of genes downstream of EGFR, both related to cellular proliferation (ERK and p38) and to apoptosis resistance (AKT and p70S6K). Finally we observed a significant activation of HIF-1\u3b1 and of its downstream genes. Another important aspect of cellular adaptation to hypoxia is the metabolic switch between oxidative and glycolytic metabolism, the so-called \u201cPasteur effect\u201d. In the present study we found that the glycolytic enzymes Glucose transporter Glut1, the Aldolase and the GAPDH were significantly enhanced in the cyanotic group which in turn demonstrates that the myocardium of patients affected by cyanogen cardiac defects is metabolic adapted to chronic hypoxia. In conclusion, the results of this PhD project support the hypothesis of a physiological role of NEU3 in mediating cellular response to hypoxic stress. It is interesting to underline that NEU3 activation is mediated by ganglioside GM3 on cellular membrane. Indeed, an increase in NEU3 level determines a reduction of GM3, which is a well know inhibitor of EGF receptor. On these premises, to mimic the effects of NEU3 activation, it could be possible to inhibit GM3 synthesis, in example by the selective inhibition of the sialyltransferase involved in the last passage of its synthesis. In this direction, our laboratory is performing some experiments with small chemical molecules, designed for blocking selectively the GM3 synthesis with the aim of activating the endogenous response to hypoxic stress

Velocity and magnetic fields within 1000 AU from a massive YSO

We want to study the velocity and magnetic field morphology in the vicinity (<1000 AU) of a massive young stellar object (YSO), at very high spatial resolution (10-100 AU). We performed milli-arcsecond polarimetric observations of the strong CH3OH maser emission observed in the vicinity of an O-type YSO, in G023.01-00.41. We have combined this information with the velocity field of the CH3OH masing gas previously measured at the same angular resolution. We analyse the velocity and magnetic fields in the reference system defined by the direction of the molecular outflow and the equatorial plane of the hot molecular core at its base, as recently observed on sub-arcsecond scales. We provide a first detailed picture of the gas dynamics and magnetic field configuration within a radius of 2000 AU from a massive YSO. We have been able to reproduce the magnetic field lines for the outer regions (>600 AU) of the molecular envelope, where the magnetic field orientation shows a smooth change with the maser cloudlets position (0.2 degree/AU). Overall, the velocity field vectors well accommodate with the local, magnetic field direction, but still show an average misalignment of 30 degrees. We interpret this finding as the contribution of a turbulent velocity field of about 3.5 km/s, responsible for braking up the alignment between the velocity and magnetic field vectors. We do resolve different gas flows which develop both along the outflow axis and across the disk plane, with an average speed of 7 km/s. In the direction of the outflow axis, we establish a collimation of the gas flow, at a distance of about 1000 AU from the disk plane. In the disk region, gas appears to stream outward along the disk plane for radii greater than 500-600 AU, and inward for shorter radii.Comment: 7 pages, 4 figures, 1 table, accepted by Astronomy & Astrophysic

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&