arXiv subjects

S. Sciortino

Publications and source records attributed to S. Sciortino.

At least 91 records · Page 5Linked to original sources

arXiv2006

The onset of X-ray emission in young stellar objects: a Chandra observation of the Serpens star-forming region

AIMS: To study the properties of X-ray emissions from young stellar objects (YSOs), through their evolution from Class I to Class III and determine whether Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally suited for this type of investigation, being populated by a dense and extremely young cluster whose members are found in all different evolutionary stages, including six well studied Class 0 sources. RESULTS: None of the six Class 0 protostars is detected in our observations, excluding the presence of sources with X-ray luminosities > 0.4 10^30 erg/s (for column densities of the order of 4 10^{23} cm^-2, or A_V ~ 200). A total of 85 X-ray sources are detected and the light curves and spectra of 35 YSOs are derived. There is a clear trend of decreasing absorbing column densities as one moves from Class I to Class III sources, and, possibly, evidence of decreasing plasma temperatures, too. We observe a strong, long-duration, flare from a Class II low-mass star, for which we derive a flaring loop length of the order of 20 stellar radii. We interpret the flaring event as originating from a magnetic flux tube connecting the star to its circumstellar disk. The presence of such a disk is supported by the detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission.

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arXiv2006

Correlation between the spatial distribution of the circumstellar disks and the massive stars in the open cluster NGC 6611. Compiled catalog and cluster parameters

Context: the observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale. Aims: we study whether there exists a correlation between the spatial distribution of stars with circumstellar disks and the position of massive stars with spectral class earlier than B5, in the open cluster NGC 6611. Methods: we created a multiband catalog of the cluster, down to V~23, using optical data from a WFI observation at 2.2m of ESO in the BVI bands, the 2MASS public point source catalog and an archival X-ray observation made with CHANDRA/ACIS. We selected the stars with infrared excess (due to the emission of a circumstellar disk) using suitable color indices independent of extinction, and studied their spatial distribution. Results: the spatial distribution of the stars with K band excess (due to the presence of a circumstellar disk) is anti correlated with that of the massive stars: the disks are more frequent at large distances from these stars. We argue that this is in agreement with the hypothesis that the circumstellar disks are heated by the UV radiation from the massive stars and photoevaporated.

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arXiv2006

The stellar content of the XMM-Newton Bright Serendipitous Survey

Context: The comparison of observed counts in a given sky direction with predictions by Galactic models yields constraints to the spatial distribution and the stellar birthrate of young stellar populations. In this work we present the results of the analysis of the stellar content of the XMM-Newton Bright Serendipitous Survey (XBSS). This unbiased survey includes a total of 58 stellar sources selected in the 0.5 -- 4.5 keV energy band, having a limiting sensitivity of $10^{-2}$ cnt s$^{-1}$ and covering an area of 28.10 sq. deg. Aims: Our main goal is to understand the recent star formation history of the Galaxy in the vicinity of the Sun. Methods: We compare the observations with the predictions obtained with XCOUNT, a model of the stellar X-ray content of the Galaxy. The model predicts the number and properties of the stars to be observed in terms of magnitude, colour, population and $f_\mathrm{x}/f_\mathrm{v}$ ratio distributions of the coronal sources detected with a given instrument and sensitivity in a specific sky direction. Results: As in other shallow surveys, we observe an excess of stars not predicted by our Galaxy model. Comparing the colours of the identified infrared counterparts with the model predictions, we observe that this excess is produced by yellow (G+K) stars. The study of the X-ray spectrum of each source reveals a main population of stars with coronal temperature stratification typical of intermediate-age stars. As no assumptions have been made for the selection of the sample, our results must be representative of the entire Solar Neighbourhood. Some stars show excess circumstellar absorption indicative of youth.

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arXiv2006

Unveiling the Cygnus OB2 stellar population with Chandra

The aim of this work is to identify the so far unknown low mass stellar population of the ~2Myr old Cygnus OB2 region, and to investigate the X-ray and near-IR stellar properties of its members. We analyzed a 97.7 ksec Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. X-ray variability ans spectral analysis of sources was characterized through the KS-test and XSPEC thermal models, respectively. We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts associated with Cygnus OB2 members. We estimate a typical absorption toward Cygnus OB2 of Av~7.0 mag. Although the region is young, very few stars (~4.4 %) show disk-induced excesses in the near-IR. X-ray variability is detected in ~13 % of the sources. Flares account for at least 60 % of the variability. O- that early B-type stars are not significantly variable. Typical X-ray spectral parameters are log(Nh)~22.25 and kT~1.35 keV. Variable and flaring sources have harder spectra with median kT=3.3 and 3.8 keV, respectively. OB stars are typically softer (kT~0.75 keV). X-ray luminosities range between 1E+30 and 1E+31 erg/s for intermediate- and low-mass stars, and 2.5x10^30 and between 6.3E+33 erg/s for OB stars. The Cygnus OB2 region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce.

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arXiv2006

XMM-Newton survey of two Upper Scorpius regions

We study X-ray emission from young stars by analyzing deep XMM-Newton observations of two regions of the Upper Scorpius association, having an age of 5 Myr. Based on near infrared and optical photometry we identify 22 Upper Scorpius photometric members among the 224 detected X-ray sources. We derive coronal properties of Upper Scorpius stars by performing X-ray spectral and timing analysis. The study of four strong and isolated stellar flares allows us to derive the length of the flaring loops. Among the 22 Upper Scorpius stars, 13 are identified as Upper Scorpius photometric members for the first time. The sample includes 7 weak-line T Tauri stars and 1 classical T Tauri star, while the nature of the remaining sources is unknown. Except for the intermediate mass star HD 142578, all the detected USco sources are low mass stars of spectral type ranging from G to late M. The X-ray emission spectrum of the most intense Upper Scorpius sources indicates metal depleted plasma with temperature of ~10 MK, resembling the typical coronal emission of active main sequence stars. At least 59% of the detected members of the association have variable X-ray emission, and the flaring coronal structures appear shorter than or comparable to the stellar radii already at the Upper Scorpius age. We also find indication of increasing plasma metallicity (up to a factor 20) during strong flares. We identify a new galaxy cluster among the 224 X-ray source detected: the X-ray spectrum of its intra cluster medium indicates a redshift of 0.41+/-0.02.

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arXiv2006

ACIS-I observations of NGC2264. Membership and X-ray properties of PMS stars

We analyze a deep, 100 ksec long, Chandra ACIS observation of NGC2264. We detect 420 sources, 85% of which are associated with known optical and NIR counterparts. More than 90% of these counterparts are NGC2264 members, thus significantly increasing the known low mass cluster population by about 100 objects. Among the sources without counterpart, ~50% are likely members, several of which we expect to be previously unknown protostellar objects. We confirm several previous findings on the X-ray activity of low mass PMS stars: X-ray luminosity is related to stellar mass, although with a large scatter; L_X/L_bol is close to but almost invariably below the saturation level, 1e-3. A comparison between CTTS and WTTS shows several differences: CTTS are on average less active than WTTS; their emission may also be more time variable and is on average slightly harder. However, we find evidence in some CTTS of extremely cool, ~0.1-0.2 keV, plasma which we speculate is heated by accretion shocks. We conclude that activity in low mass PMS stars, while generally similar to that of saturated MS stars, may be affected by mass accretion in several ways: accretion is likely responsible for very soft X-ray emission directly produced in the accretion shock; it may reduce the average energy output of solar-like coronae, at the same time making them hotter and more dynamic.

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arXiv2006

X-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551

We present the results of a five-day monitoring campaign with XMM-Newton of six X-ray bright young stellar objects (YSOs) in the star-forming complex L1551 in Taurus. All stars present significant variability on the five-day time scale. Modulation of the light curve on time scales comparable with the star's rotational period appeared to be present in the case of one weak-lined T Tauri star. Significant spectral variations between the 2000 and the 2004 observations were detected in the (unresolved) classical T Tauri binary system XZ Tau: a hot plasma component which was present in the X-ray spectrum in 2000 had significantly weakened in 2004. As XZ Tau N was undergoing a strong optical outburst in 2000, which had terminated since then, we speculate on the possible relationship between episodic, burst accretion, and X-ray heating. The transition object HL Tau underwent a strong flare with a complex temperature evolution, which is indicative of an event confined within a very large magnetic structure (few stellar radii), similar to the ones found in YSOs in the Orion Nebula Cluster.

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arXiv2006

The discovery of an expanding X-ray source in the HH 154 protostellar jet

We have performed, in October 2005, a deep Chandra X-ray observation of HH 154. Comparison with the previous (2001) Chandra observation allows to detect proper motion down to the level predicted by models of X-ray emitting shocks in the jet. The 2005 Chandra observation of HH 154 shows unexpected morphological changes of the X-ray emission in comparison with the 2001 data. Two components are present: a stronger, point-like component with no detectable motion and a weaker component which has expanded in size by approximately 300 AU over the 4 years time base of the two observations. This expansion corresponds to approximately 500 km/s, very close to the velocity of the X-ray emitting shock in the simple theoretical models. The 2005 data show a more complex system than initially thought (and modeled), with multiple components with different properties. The observed morphology is possibly indicating a pulsed jet propagating through a non-homogeneous medium, likely with medium density decreasing with distance from the driving source. Detailed theoretical modeling and deeper X-ray observations will be needed to understand the physics of this fascinating class of sources.

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arXiv2006

Chandra X-Ray Observations of Young Clusters. III. NGC 2264 and the Orion Flanking Fields

Chandra observations of solar-like pre-main sequence (PMS) stars in the Orion Flanking Fields (age ~1 Myr) and NGC 2264 (~3 Myr) are compared with the results of the COUP survey of similar objects in the ONC (~0.5 Myr). The correlations between log Lx and mass found for PMS stars on convective tracks in these clusters are consistent with the relationships found for the ONC, indicating little change in the median values of either log Lx or log Lx/Lbol during the first ~3-5 Myr of evolution down convective tracks. The fraction of stars with extreme values of Lx, more than 10 times higher than the average for a given Lbol or with log Lx/Lbol greater than the canonical saturation value of -2.9, is however larger by a factor of two in the younger ONC when compared with the Orion FF and NGC 2264. PMS stars in NGC 2264 on radiative tracks have Lx/Lbol values that are systematically lower by a factor of ~10 times than those found for stars of similar mass on convective tracks. The dramatic decrease in flux from convective to radiative phases of PMS evolution is likely related to major structural changes which influence the efficiency of magnetic field generation and thus the level of magnetic activity. As in the ONC, we find that stars with measured periods have, on average, higher X-ray luminosities. However, there is a wide range in Lx and Lx/Lbol for both periodic and non-periodic stars of similar mass. Among stars with measured periods, the level of X-ray emission does not correlate with the rotation rate. For this data set, we find no statistically significant correlation between X-ray flux and (a) the presence or absence of circumstellar accretion disks, or (b) disk accretion rates as assessed from ultraviolet excesses.

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arXiv2006

Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton

We report on a deep X-ray survey of the young (~ 140 Myr), rich open cluster NGC 2516 obtained with the EPIC camera on board the XMM-Newton satellite. By combining data from six observations, a high sensitivity, greater than a factor 5 with respect to recent Chandra observations, has been achieved. We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC 2516 Main Sequence. The X-ray luminosities of cluster stars span the range log Lx (erg/s) = 28.4 - 30.8. The representative temperatures span the 0.3 - 0.6 keV (3.5 - 8 MK) range for the cool component and 1.0 - 2.0 keV (12 -- 23 MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC 2391, and Blanco 1. While no significant differences are found in X-ray spectra, NGC 2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey evidence the lack of variability amplitudes larger than a factor 2 in solar type stars in a ~11 yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or different by period and amplitude in young stars.

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arXiv2005

Rotational modulation of X-ray emission in Orion Nebula young stars

We investigate the spatial distribution of X-ray emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light-curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17x17 arcmin field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period while in seven cases it is about half that value, possibly due to the presence of X-ray emitting structures at opposite stellar longitudes. These results constitute the largest sample of low mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (L_X/L_bol ~ 10^(-3)) showing that saturation is not due to the filling of the stellar surface with X-ray emitting regions.

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arXiv2005

Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic fields?

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 days observation of the Orion Nebula Cluster (ONC). Analysis of the flare decay allows to determine the size, peak density and magnetic field of the flaring structure. A total of 32 events (the most powerful 1% of COUP flares), have sufficient statistics for the analysis. A broad range of decay times (from 10 to 400 ks) are present in the sample. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures which are found, are in a number of cases very long, with semi-lengths up to 10^12 cm, implying the presence of magnetic fields of hundreds of G extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures ($ >> R_*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L <= R_*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures which confine the flaring plasma are actually the same type of structures which channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

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arXiv2005

X-ray emission from early-type stars in the Orion Nebula Cluster

The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Only two of the massive stars show exclusively the constant soft-spectrum emission expected from the standard model for X-ray emission from hot stars involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity, suggesting magnetic confinement of winds with large-scale shocks. Most of the stars in the weak-wind class exhibit X-ray flares and have luminosities that are consistent with magnetic activity from known or unseen low-mass companions. All non-detections belong to the weak-wind class.

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arXiv2005

COUP census of X-ray stars in BN-KL and OMC-1S

We present a study of the X-ray sources detected in the vicinity of two density peaks in the Orion Molecular Cloud 1 (OMC-1) behind the Orion Nebula Cluster (ONC), as seen in the exceptionally deep (~10 days) exposure of the Chandra Orion Ultradeep Project (COUP). We focus on a 40"*50" region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL) and a 60"*75" region around OMC-1S, a secondary star-forming peak some 90" south of BN-KL. Forty-three and sixty X-ray sources were detected in BN-KL and OMC-1S, respectively, of which half and one-third, respectively, were found to be foreground members of the ONC, while the remaining sources are identified as obscured X-ray sources with column densities 22<log(NH)/cm^{-2})<24. All but 5 and 18 of these sources have near-infrared stellar counterparts, and 22 of these appear to be young stellar objects (YSOs). X-ray sources are found close to four luminous mid-IR sources namely BN, IRc3-i2, IRc2-C, and Source n; their X-ray variability and spectral properties are typical of coronal activity in low-mass stars rather than wind emission from massive stars, suggesting that the X-ray emission may be arising from companions. The X-ray light curve of the X-ray source close to BN shows a periodicity of ~8.3 days and from an X-ray image deconvolution of the region around BN, we conclude that either BN itself or a low mass companion with a projected separation of ~200 AU was detected. On the other hand, no emission is seen from the bright radio Source I, held by some to be the main source of luminosity in BN-KL. In OMC-1S, Chandra unveils a new subcluster of seven YSOs without infrared counterparts (abridged).

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arXiv2004

A survey for Fe 6.4 keV emission in young stellar objects in rho Oph: the strong fluorescence from Elias 29

We report the results of a search for 6.4 keV Fe fluorescent emission in Young Stellar Objects (YSOs) with measured accretion luminosities in the rho Oph cloud, using all existing chandra and XMM-Newton observations of the region. A total of nine such YSOs have X-ray data with sufficiently high S/N for the 6.4 keV line to be potentially detected if present. A positive detection of the Fe 6.4 keV line is reported for one object, Elias 29, in both the XMM-Newton and the chandra data. The 6.4 keV line is detected in Elias 29 both during quiescent and flaring emission, unlikely all previously reported detections of 6.4 keV Fe fluorescence in YSOs which were made during intense flaring. The observed equivalent width of the fluorescent line is large, at W_alpha approx 140 eV, ruling out fluorescence from diffuse circumstellar material. It is also larger than expected for simple reflection from a solar-composition photosphere or circumstellar disk, but it is compatible with being due to fluorescence from a centrally illuminated circumstellar disk. The X-ray spectrum of Elias 29 is also peculiar in terms of its high (ionized) Fe abundance, as evident from the very intense Fe xxv 6.7 keV line emission; we speculate on the possible mechanism leading to the observed high abundance.

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arXiv2004

High Resolution X-ray Spectroscopy of Pre-Main-Sequence Stars: TWA 5 and PZ Tel

We report on the analysis of high resolution X-ray spectra of two pre-main-sequence stars: TWA 5 (observed with XMM-Newton) and PZ Telescopii (observed with Chandra/HETGS). TWA 5 is a classical T Tauri star in the TW Hydrae association while PZ Tel is a rapidly rotating weak-lined T Tauri star in the beta-Pictoris moving group. For both stars we have reconstructed the emission measure distribution and derived the coronal abundances to check for possible patterns of the abundances related to the first ionization potential of the various elements. We have also derived estimates of the plasma density from the analysis of the He-like triplets. We compare the characteristics of our targets with those of other pre-main sequence stars previously analyzed by other authors: TW Hya, HD 98800 and HD 283572. Our findings suggest that X-ray emission from classical T Tauri and weak-lined T Tauri stars is produced in all cases by magnetically-heated coronae, except for TW Hya which has unique plasma temperatures and densities. Moreover we derive that TWA 5 has the same peculiar Ne/Fe abundance ratio as TW Hya.

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arXiv2004

Chandra Orion Ultradeep Project: Observations and Source Lists

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.

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arXiv2004

X-ray spectral and timing characteristics of the stars in the young open cluster IC 2391

We present X-ray spectral and timing analysis of members of the young open cluster IC 2391 observed with the XMM-Newton observatory. We detected 99 X-ray sources by analysing the summed data obtained from MOS1, MOS2 and pn detectors of the EPIC camera; 24 of them are members, or probable members, of the cluster. Stars of all spectral types have been detected, from the early-types to the late-M dwarfs. Despite the capability of the instrument to recognize up to 3 thermal components, the X-ray spectra of the G, K and M members of the cluster are well described with two thermal components (at kT$_1 \sim$ 0.3-0.5 keV and kT$_2 \sim$ 1.0-1.2 keV respectively) while the X-ray spectra of F members require only a softer 1-T model. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that approximately 46% of the members of IC 2391 are variable with a confidence level $>$99%. The comparison of our data with those obtained with ROSAT/PSPC, nine years earlier, and ROSAT/HRI, seven years earlier, shows that there is no evidence of significant variability on these time scales, suggesting that long-term variations due to activity cycles similar to that on the Sun are not common, if present at all, among these young stars.

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