Constraining the spatial and thermal structure of the gaseous component of circumstellar disks is crucial to understand star and planet formation. Models predict that the [Ne II] line at 12.81 {mu}m detected in young stellar objects with Spitzer trac
es disk gas and its response to high energy radiation, but such [Ne II] emission may also originate in shocks within powerful outflows. To distinguish between these potential origins for mid-infrared [Ne II] emission and to constrain disk models, we observed 32 young stellar objects using the high resolution (R~30000) mid-infrared spectrograph VISIR at the VLT. We detected the 12.81 {mu}m [Ne II] line in 12 objects, tripling the number of detections of this line in young stellar objects with high spatial and spectral resolution spectrographs. We obtain the following main results: a) In Class I objects the [Ne II] emission observed from Spitzer is mainly due to gas at a distance of more than 20-40 AU from the star, where neon is, most likely, ionized by shocks due to protostellar outflows. b) In transition and pre-transition disks, most of the emission is confined to the inner disk, within 20-40 AU from the central star. c) Detailed analysis of line profiles indicates that, in transition and pre-transition disks, the line is slightly blue-shifted (2-12 km s{^-1}) with respect to the stellar velocity, and the line width is directly correlated with the disk inclination, as expected if the emission is due to a disk wind. d) Models of EUV/X-ray irradiated disks reproduce well the observed relation between the line width and the disk inclination, but underestimate the blue-shift of the line.
The soft X-ray emission from high density plasma in CTTS is associated with the accretion process. It is still unclear whether this high density cool plasma is heated in the accretion shock, or if it is coronal plasma fed/modified by the accretion pr
ocess. We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph (Chandra/HETGS data to constrain the X-ray emitting plasma components, and optical observations to constrain the characteristics of accretion and magnetic field). We analyze a 200 ks Chandra/HETGS observation of V2129 Oph, subdivided into two 100 ks segments, corresponding to two different phases within one stellar rotation. The X-ray emitting plasma covers a wide range of temperatures: 2-34 MK. The cool plasma component of V2129 Oph varies between the two segments of the Chandra observation: high density plasma (log Ne ~ 12.1) with high EM at ~ 3-4 MK is present during the 1st segment; during the 2nd segment this plasma component has lower EM and lower density (log Ne < 11.5), although the statistical significance of these differences is marginal. Hotter plasma components, T > 10 MK, show variability on short time scales (~ 10 ks), typical of coronal plasma. A clear flare, detected in the 1st segment, could be located in a large coronal loop (> 3 Rstar). Our observation provides further confirmation that the dense cool plasma at a few MK in CTTS is material heated in the accretion shock. The variability of this cool plasma component on V2129 Oph may be explained in terms of X-rays emitted in the accretion shock and seen with different viewing angles at the two rotational phases probed by our observation. During the 1st time interval direct view of the shock region is possible, while, during the 2nd, the accretion funnel itself intersects the line of sight to the shock region, preventing us from observing accretion-driven X-rays.
The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well understood since it is very hard to observe YSOs in the protostellar phase. We study the X-ray properties of Class 0-I objects in the Orion Nebula Cluster (O
NC) and compare them with those of the more evolved Class II and III members. Using Chandra Orion Ultradeep Project (COUP) data, we study the X-ray properties of stars in different evolutionary classes: luminosities, NH, temperatures and time variability are compared in order to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8 microns using data from HST, [email protected] ESO and ISPI@4m CTIO telescopes, and Spitzer IRAC. We select high probability candidate Class 0-I protostars, distinguishing between those having a spectral energy distribution which rises from K up to 8 microns (Class 0-Ia) from those where the SED rises from K up to 4.5 microns and decreasing afterwards (Class 0-Ib). We select a sample of bona fide Class II stars and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, both in the total and hard bands, than the more evolved Class II stars with mass larger than 0.5 Msun; these latter show X-ray luminosities similar to those of Class 0-Ib stars. This result supports the hypothesis that the onset of X-ray emission occurs at a very early stage of star formation. Temporal variability and spectral properties of Class 0-I stars are similar to those of the more evolved Class II and III objects, except for a larger absorption likely due to gas in the circumstellar material.
Aims: We characterize individual and ensemble properties of X-ray flares from stars in the CygOB2 and ONC star-forming regions. Method: We analyzed X-ray lightcurves of 1003 CygOB2 sources observed with Chandra for 100 ksec and of 1616 ONC sources de
tected in the ``Chandra Orion Ultra-deep Project 850 ksec observation. We employed a binning-free maximum likelihood method to segment the light-curves into intervals of constants signal and identified flares on the basis of both the amplitude and the time-derivative of the source luminosity. We then derived and compared the flare frequency and energy distribution of CygOB2 and ONC sources. The effect of the length of the observation on these results was investigated by repeating the statistical analysis on five 100 ksec-long segments extracted from the ONC data. Results: We detected 147 and 954 flares from the CygOB2 and ONC sources, respectively. The flares in CygOB2 have decay times ranging from ~0.5 to about 10 hours. The flare energy distributions of all considered flare samples are described at high energies well by a power law with index alpha=-(2.1+-0.1). At low energies, the distributions flatten, probably because of detection incompleteness. We derived average flare frequencies as a function of flare energy. The flare frequency is seen to depend on the sources intrinsic X-ray luminosity, but its determination is affected by the length of the observation. The slope of the high-energy tail of the energy distribution is, however, affected little. A comparison of CygOB2 and ONC sources, accounting for observational biases, shows that the two populations, known to have similar X-ray emission levels, have very similar flare activity.
Context. X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass star
s. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC). Aims. Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 M_sun) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. Methods. We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares. Results. The high energy tail of the energy distribution of flares is well described by a power-law with index 2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data. Comparing low mass stars with stars in the mass interval (0.9-1.2M_sun), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies. Conclusions. Our observational results are consistent with the following model/scenario: the light curves are entirely built by over- lapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.
