The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares

Most stellar flares' soft X-ray lightcurves possess a `typical' morphology, which consists of a rapid rise followed by a slow exponential decay. However, a study of 216 of the brightest flares on 161 pre-main sequence stars, observed during the Chandra Orion-Ultradeep Project (COUP), showed that many flare lightcurves depart from this typical morphology. While this can be attributed to the superposition of multiple typical flares, we explore the possibility that the time-variable eclipsing of flares by their host stars may also be an important factor. We assume each flare is contained within a single, uniform plasma density magnetic loop and specify the intrinsic variation of the flare's emission measure with time. We consider rotational eclipse by the star itself, but also by circumstellar discs and flare-associated prominences. Based on this simple model, we generate a set of flares similar to those observed in the COUP database. Many eclipses simply reduce the flare's maximum emission measure or decay time. We conclude therefore that eclipses often pass undetected, but usually have only a modest influence on the flare emission measure profile and hence the derived loop lengths. We show that eclipsing can easily reproduce the observed atypical flare morphologies. The number of atypical modelled flare morphologies is however much less than that found in the COUP sample. The large number of observed atypical flare morphologies, therefore, must be attributed to other processes such as multiple flaring loops.Comment: 11 pages, 9 figure

New distance measures for classifying X-ray astronomy data into stellar classes

The classification of the X-ray sources into classes (such as extragalactic sources, background stars, ...) is an essential task in astronomy. Typically, one of the classes corresponds to extragalactic radiation, whose photon emission behaviour is well characterized by a homogeneous Poisson process. We propose to use normalized versions of the Wasserstein and Zolotarev distances to quantify the deviation of the distribution of photon interarrival times from the exponential class. Our main motivation is the analysis of a massive dataset from X-ray astronomy obtained by the Chandra Orion Ultradeep Project (COUP). This project yielded a large catalog of 1616 X-ray cosmic sources in the Orion Nebula region, with their series of photon arrival times and associated energies. We consider the plug-in estimators of these metrics, determine their asymptotic distributions, and illustrate their finite-sample performance with a Monte Carlo study. We estimate these metrics for each COUP source from three different classes. We conclude that our proposal provides a striking amount of information on the nature of the photon emitting sources. Further, these variables have the ability to identify X-ray sources wrongly catalogued before. As an appealing conclusion, we show that some sources, previously classified as extragalactic emissions, have a much higher probability of being young stars in Orion Nebula.Comment: 29 page

Gaia Stellar Kinematics in the Head of the Orion A Cloud: Runaway Stellar Groups and Gravitational Infall

This work extends previous kinematic studies of young stars in the Head of the Orion A cloud (OMC-1/2/3/4/5). It is based on large samples of infrared, optical, and X-ray selected pre-main sequence stars with reliable radial velocities and Gaia-derived parallaxes and proper motions. Stellar kinematic groups are identified assuming they mimic the motion of their parental gas. Several groups are found to have peculiar kinematics: the NGC 1977 cluster and two stellar groups in the Extended Orion Nebula (EON) cavity are caught in the act of departing their birthplaces. The abnormal motion of NGC 1977 may have been caused by a global hierarchical cloud collapse, feedback by massive Ori OB1ab stars, supersonic turbulence, cloud-cloud collision, and/or slingshot effect; the former two models are favored by us. EON groups might have inherited anomalous motions of their parental cloudlets due to small-scale `rocket effects' from nearby OB stars. We also identify sparse stellar groups to the east and west of Orion A that are drifting from the central region, possibly a slowly expanding halo of the Orion Nebula Cluster. We confirm previously reported findings of varying line-of-sight distances to different parts of the cloud's Head with associated differences in gas velocity. Three-dimensional movies of star kinematics show contraction of the groups of stars in OMC-1 and global contraction of OMC-123 stars. Overall, the Head of Orion A region exhibits complex motions consistent with theoretical models involving hierarchical gravitational collapse in (possibly turbulent) clouds with OB stellar feedback.Comment: Accepted for publication in MNRAS. 26 pages, 13 figures. The two 3-D stellar kinematic movies, aimed as Supplementary Materials, can be found on YouTube at: https://youtu.be/B4GHCVvCYfo (`restricted' sample) and https://youtu.be/6fUu8sP0QFI (`full' sample

The Chamaeleon II low-mass star-forming region: radial velocities, elemental abundances, and accretion properties

Radial velocities, elemental abundances, and accretion properties of members of star-forming regions (SFRs) are important for understanding star and planet formation. While infrared observations reveal the evolutionary status of the disk, optical spectroscopy is fundamental to acquire information on the properties of the central star and on the accretion characteristics. 2MASS archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have provided disk properties of a large number of young stars. We complement these data with spectroscopy with the aim of providing physical stellar parameters and accretion properties. We use FLAMES/UVES+GIRAFFE observations of 40 members of Cha II to measure radial velocities through cross-correlation technique, Li abundances by means of curves of growth, and for a suitable star elemental abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the equivalent widths of the Halpha, Hbeta, and the HeI-5876, 6678, 7065 Angstrom emission lines, we estimate the mass accretion rates, dMacc/dt, for all the objects. We derive a radial velocity distribution for the Cha II stars (=11.4+-2.0 km/s). We find dMacc/dt prop. to Mstar^1.3 and to Age^(-0.82) in the 0.1-1.0 Msun mass regime, and a mean dMacc/dt for Cha II of ~7*10^(-10) Msun/yr. We also establish a relationship between the HeI-7065 Angstrom line emission and the accretion luminosity. The radial velocity distributions of stars and gas in Cha II are consistent. The spread in dMacc/dt at a given stellar mass is about one order of magnitude and can not be ascribed entirely to short timescale variability. Analyzing the relation between dMacc/dt and the colors in Spitzer and 2MASS bands, we find indications that the inner disk changes from optically thick to optically thin at dMacc/dt~10^(-10) Msun/yr. Finally, the disk fraction is consistent with the age of Cha II.Comment: 21 Pages, 15 Figures, 7 Tables. Accepted for publication in Astronomy and Astrophysics. Abstract shortene

On the accretion properties of young stellar objects in the L1615/L1616 cometary cloud

We present the results of FLAMES/UVES and FLAMES/GIRAFFE spectroscopic observations of 23 low-mass stars in the L1615/L1616 cometary cloud, complemented with FORS2 and VIMOS spectroscopy of 31 additional stars in the same cloud. L1615/L1616 is a cometary cloud where the star formation was triggered by the impact of the massive stars in the Orion OB association. From the measurements of the lithium abundance and radial velocity, we confirm the membership of our sample to the cloud. We use the equivalent widths of the H$\alpha$, H$\beta$, and the HeI $\lambda$5876, $\lambda$6678, $\lambda$7065 \AA$$emission lines to calculate the accretion luminosities, $L_{\rm acc}$, and the mass accretion rates, $\dot M_{\rm acc}$. We find in L1615/L1616 a fraction of accreting objects ($\sim 30\%$), which is consistent with the typical fraction of accretors in T associations of similar age ($\sim 3$ Myr). The mass accretion rate for these stars shows a trend with the mass of the central object similar to that found for other star-forming regions, with a spread at a given mass which depends on the evolutionary model used to derive the stellar mass. Moreover, the behavior of the $2MASS/WISE$ colors with $\dot M_{\rm acc}$ indicates that strong accretors with $\log \dot M_{\rm acc} \gt -8.5$ dex show large excesses in the $JHK{\rm s}$ bands, as in previous studies. We also conclude that the accretion properties of the L1615/L1616 members are similar to those of young stellar objects in T associations, like Lupus.Comment: Accepted by Astronomy and Astrophysics. 17 pages, 11 figures, 6 table

X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks

We study the properties of powerful X-ray flares from 161 pre-main sequence (PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula region. Relationships between flare properties, protoplanetary disks and accretion are examined in detail to test models of star-disk interactions at the inner edge of the accretion disks. Previous studies had found no differences in flaring between diskfree and accreting systems other than a small overall diminution of X-ray luminosity in accreting systems. The most important finding is that X-ray coronal extents in fast-rotating diskfree stars can significantly exceed the Keplerian corotation radius, whereas X-ray loop sizes in disky and accreting systems do not exceed the corotation radius. This is consistent with models of star-disk magnetic interaction where the inner disk truncates and confines the PMS stellar magnetosphere. We also find two differences between flares in accreting and diskfree PMS stars. First, a subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are preferentially present in accreting systems. Second, we tentatively find that accreting stars produce flares with shorter durations. Both results may be consequences of the distortion and destabilization of the stellar magnetosphere by the interacting disk. Finally, we find no evidence that any flare types, even slow-rise flat-top flares are produced in star-disk magnetic loops. All are consistent with enhanced solar long-duration events with both footprints anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2 table

VST processing facility: first astronomical applications

VST--Tube is a new software package designed to process optical astronomical images. It is an automated pipeline to go from the raw exposures to fully calibrated co-added images, and to extract catalogs with aperture and PSF photometry. A set of tools allow the data administration and the quality check of the intermediate and final products. VST-Tube comes with a Graphical User Interface to facilitate the interaction between data and user. We outline here the VST--Tube architecture and show some applications enlightening some of the characteristics of the pipeline.Comment: Presented to the 54th Congress SAIt, 4-7 May 2010, Naples, Ital

The nature of the fluorescent iron line in V 1486 Ori

The fluorescent 6.4 keV iron line provides information on cool material in the vicinity of hard X-ray sources as well as on the characteristics of the X-ray sources themselves. First discovered in the X-ray spectra of the flaring Sun, X-ray binaries and active galactic nuclei (AGN), the fluorescent line was also observed in a number of stellar X-ray sources. The young stellar object (YSO) V1486 Ori was observed in the framework of the Chandra Ultra Deep Project (COUP) as the source COUP 331. We investigate its spectrum, with emphasis on the strength and time variability of the fluorescent iron K-alpha line, derive and analyze the light curve of COUP 331 and proceed with a time-resolved spectral analysis of the observation. The light curve of V 1486 Ori shows two major flares, the first one lasting for (approx) 20 ks with a peak X-ray luminosity of 2.6*10^{32} erg/s (dereddened in the 1-10 keV band) and the second one -- only partially observed -- for >60 ks with an average X-ray luminosity of 2.4*10^{31} erg/s (dereddened). The spectrum of the first flare is very well described by an absorbed thermal model at high temperature, with a pronounced 6.7 keV iron line complex, but without any fluorescent K-alpha line. The X-ray spectrum of the second flare is characterized by even higher temperatures (>= 10 keV) without any detectable 6.7 keV Fe XXV feature, but with a very strong fluorescent iron K-alpha line appearing predominantly in the 20 ks rise phase of the flare. Preliminary model calculations indicate that photoionization is unlikely to account for the entire fluorescent emission during the rise phase.Comment: 4 pages, letter, accepted for publication in A&