arXiv subjects

F. Favata

Publications and source records attributed to F. Favata.

At least 55 records · Page 3Linked to original sources

arXiv2005

X-rays from accretion shocks in T Tauri stars: The case of BP Tau

We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O {\sc vii} triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.

astro-ph↗

arXiv2004

Iron Fluorescent Line Emission from Young Stellar Objects in the Orion Nebula

We present the result of a systematic search for the iron Kalpha fluorescent line at 6.4 keV among 1616 X-ray sources detected by ultra-deep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultra-deep Project (COUP). Seven sources are identified to have an excess emission at 6.4 keV among 127 control sample sources with significant counts in the 6.0-9.0 keV band. These seven sources are young stellar objects (YSOs) characterized by intense flare-like flux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the fluorescence by interstellar or circumstellar matter along the line of sight. The X-ray spectral fits and NIR colors of the 6.4 keV sources show that these sources have X-ray absorption of > 1x10^22 cm^(-2) and NIR excess emission, which is not expected when the fluorescence occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises from reflection off of circumstellar disks, which are irradiated by the hard X-ray continuum emission of magnetic reconnection flares.

astro-ph↗

arXiv2004

Accretion, fluorescent X-ray emission and flaring magnetic structures in YSOs

I present some recent developments on high-energy phenomena in YSOs, concentrating on the new evidence for accretion-induced X-ray emission in YSOs, for Fe 6.4 keV fluorescent emission from the disks of YSOs and for very long magnetic structures responsible for intense X-ray flares, likely connecting the star and the circumstellar disk.

astro-ph↗

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.

astro-ph↗

arXiv2004

Inferring coronal structure from X-ray lightcurves and Doppler shifts: a Chandra study of AB Doradus

The Chandra X-ray observatory monitored the single cool star, AB Doradus, continuously for a period lasting 88 ksec (1.98 Prot) in 2002 December with the LETG/HRC-S. The X-ray lightcurve shows rotational modulation, with three peaks that repeat in two consecutive rotation cycles. These peaks may indicate the presence of compact emitting regions in the quiescent corona. Centroid shifts as a function of phase in the strongest line profile, O VIII 18.97 A, indicate Doppler rotational velocities with a semi-amplitude of 30 +/- 10 km/s. By taking these diagnostics into account along with constraints on the rotational broadening of line profiles (provided by archival Chandra HETG Fe XVII and FUSE Fe XVIII profile) we can construct a simple model of the X-ray corona that requires two components. One of these components is responsible for 80% of the X-ray emission, and arises from the pole and/or a homogeneously distributed corona. The second component consists of two or three compact active regions that cause modulation in the lightcurve and contribute to the O VIII centroid shifts. These compact regions account for 16% of the emission and are located near the stellar surface with heights of less than 0.3R*. At least one of the compact active regions is located in the partially obscured hemisphere of the inclined star, while one of the other active regions may be located at 40 degrees. High quality X-ray data such as these can test the models of the coronal magnetic field configuration as inferred from magnetic Zeeman Doppler imaging.

astro-ph↗

arXiv2004

Inferring coronal structure using X-ray spectra: A Chandra study of AB Dor

The Chandra X-ray observatory monitored the single cool star, AB Doradus, continuously for a period lasting 88ksec (1.98 Prot) in 2002 December with the LETG/HRC-S. The X-ray lightcurve shows significant rotational modulation. It can be represented as having a flat level of emission superimposed with bright flaring regions that appear at the same phases in both rotation cycles. Phase-binned OVIII line profiles show centroid shifts that also repeat in consecutive rotation cycles. These Doppler shifts trace a roughly sinusoidal pattern with a a semi-amplitude of 30 +/-10km/s. By taking both the lightcurve and spectral diagnostics into account along with constraints on the rotational broadening of line profiles (provided by archival Chandra HETG FeXVII line profiles) we can construct a simple model of the X-ray corona. The corona can be described as having two components, one component is homogeneously distributed, extending less than 1.75R*; and the other consists of at least two compact emitting regions near the stellar surface. These compact regions account for 16% of the X-ray emission and are likely to be located less than 0.4R* above the stellar surface.

astro-ph↗

arXiv2004

Chandra observations of the massive star-forming region S106: X-ray emission from the embedded massive protostellar object IRS 4

We present Chandra observations of the massive star-forming region S106, a prominent HII region in Cygnus, associated with an extended molecular cloud and a young cluster. The nebula is excited by a single young massive star located at the center of the molecular cloud and the embedded cluster. The prominence of the cluster in the Chandra observation presented here confirms its youth and allows some of its members to be studied in more detail. We detect X-ray emission from the young massive central source S106 IRS 4, the deeply embedded central object which drives the bipolar nebula with a mass loss rate approximately 1-2 orders of magnitude higher than main sequence stars of comparable luminosity. Still, on the basis of its wind momentum flux the X-ray luminosity of S106 IRS 4 is comparable to the values observed in more evolved (main sequence and giant) massive stars, suggesting that the same process which is responsible for the observed X-ray emission from older massive stars is already at work at these early stages.

astro-ph↗

arXiv2004

The photospheric abundances of active binaries III. Abundance peculiarities at high activity level

We report the determination from high-resolution spectra of the atmospheric parameters and abundances of 13 chemical species (among which lithium) in 8 single-lined active binaries. These data are combined with our previous results for 6 other RS CVn systems to examine a possible relationship between the photospheric abundance patterns and the stellar activity level. The stars analyzed are generally found to exhibit peculiar abundance ratios compared to inactive, galactic disk stars of similar metallicities. We argue that this behaviour is unlikely an artefact of errors in the determination of the atmospheric parameters or non-standard mixing processes along the red giant branch, but diagnoses instead the combined action of various physical processes related to activity. The most promising candidates are cool spot groups covering a very substantial fraction of the stellar photosphere or NLTE effects arising from nonthermal excitation. However, we cannot exclude the possibility that more general shortcomings in our understanding of K-type stars (e.g. inadequacies in the atmospheric models) also play a significant role. Lastly, we call attention to the unreliability of the (V-R) and (V-I) colour indices as temperature indicators in chromospherically active stars.

astro-ph↗

arXiv2004

The Chandra Deep Field-North Survey: XVII. Evolution of magnetic activity in old late-type stars

The extremely sensitive Chandra Deep Field-North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spindown of stellar rotation from a magnetized stellar wind. Eleven X-ray sources constitute a well-defined sample of 2 G, 2 K0-K4, and 7 M2-M5 stars with median distance around 300 pc. X-ray luminosities are typically log Lx ~ 27 erg/s and is dominated by flares rather than quiescent coronal emission. Models of the population indicates that the CDF-N stars are the most magnetically active old disk stars. A substantial decline in X-ray luminosities over the 1<t<11 Gyr age interval is required. This is the first demonstration that the coronal and flaring components of stellar magnetic activity -- and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface -- exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types and X-ray luminosities of the sample has Lx ~ 1/t^2 erg/s which is faster than the 1/t decay rate predicted from widely accepted rotational spindown rates and X-ray-activity relations.

astro-ph↗

arXiv2004

High-amplitude, long-term X-ray variability in the solar-type star HD 81809: the beginning of an X-ray activity cycle?

We present the initial results from our XMM program aimed at searching for X-ray activity cycles in solar-type stars. HD 81809 is a G2-type star (somewhat more evolved than the Sun, and with a less massive companion) with a pronounced 8.2 yr chromospheric cycle, as evident from from the Mt. Wilson program data. We present here the results from the initial 2.5 years of XMM observations, showing that large amplitude (a factor of approx. 10) modulation is present in the X-ray luminosity, with a clearly defined maximum in mid 2002 and a steady decrease since then. The maximum of the chromospheric cycle took place in 2001; if the observed X-ray variability is the initial part of an X-ray cycle, this could imply a phase shift between chromospheric and coronal activity, although the current descent into chromospheric cycle minimum is well reflected into the star's X-ray luminosity. The observations presented here provide clear evidence for the presence of large amplitude X-ray variability coherent with the activity cycle in the chromosphere in a star other than the Sun.

astro-ph↗

arXiv2004

Parameter properties and stellar population of the old open cluster NGC 3960

We present a $BVI$ photometric and astrometric catalogue of the open cluster NGC 3960, down to limiting magnitude $V\sim22$, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope at La Silla. The photometry of all the stars detected in our field of view has been used to estimate a map of the strong differential reddening affecting this area. Our results indicate that, within the region where the cluster dominates, the $E(V-I)$ values range from 0.21 up to 0.78, with $E(V-I)=0.36$ ($E(B-V)=0.29$) at the nominal cluster centroid position; color excesses $E(V-I)$ up to 1 mag have been measured in the external regions of the field of view where field stars dominate. The reddening corrected color-magnitude diagram (CMD) allows us to conclude that the cluster has an age between 0.9 and 1.4 Gyr and a distance modulus of $(V-M_V)_0=11.35$. In order to minimize field star contamination, their number has been statistically subtracted based on the surface density map. The empirical cluster main sequence has been recovered in the $V$ vs. $V-I$ and in the $J$ vs. $J-K_S$ planes, using optical and infrared data, respectively. From these empirical cluster main sequences, two samples of candidate cluster members were derived in order to obtain the luminosity distributions as a function of the $V$ and $J$ magnitudes. The Luminosity Functions have been transformed into the corresponding Mass Functions; for $M>1 M_\odot$, the two distributions have been fitted with a power law of index $α_V=2.95\pm0.53$ and $α_J=2.81\pm0.84$ in $V$ and in $J$, respectively, while the Salpeter Mass Function in this notation has index $α=2.35$.

astro-ph↗

arXiv2003

Coronal versus photospheric abundances of stars with different activity levels

We report a detailed analysis of the coronal abundance of 4 stars with varying levels of activity and with accurately known photospheric abundances. The coronal abundance is determined here using a line flux analysis and a full determination of the differential emission measure. Photospheric abundance values are taken from literature works. Previous coronal abundance determinations have generally been compared to solar photospheric abundances; from this a number of general properties have been inferred, such as the presence of a coronal metal depletion with an inverse First Ionization Potential correlated with activity level. Here we show that, when coronal abundances are compared with real photospheric values for the individual stars, the resulting pattern can be very different. Also, we present evidence that, in some cases, the coronal metal abundance may not be uniform in the corona; in particular it can vary with the temperature of the emitting plasma.

astro-ph↗

arXiv2003

Characterising stellar micro-variability for planetary transit searches

(abridged) A method for simulating light curves containing stellar micro-variability for a range of spectral types and ages, based on parameter-by-parameter scaling of a multi-component fit to the solar irradiance power spectrum (from VIRGO/PMO6 data), and scaling laws derived from ground based observations of various stellar samples, is presented. We observe a correlation in the Sun between the amplitude of the power spectrum on weeks timescales and the BBSO Ca II K-line index of chromospheric activity. On the basis of this evidence, the chromospheric activity level, predicted from rotation period and B-V colour estimates according to the relationship first introduced by Noyes(1983) and Noyes et al. (1984), is used to predict the variability power on weeks time scale. The rotation period is estimated on the basis of a fit to the distribution of rotation period versus $B-V$ observed in the Hyades and the Skumanich (1972) spin-down law. The characteristic timescale of the variability is also scaled according to the rotation period. We use this model to estimate the impact of the target star type and age on the detection capability of missions such as Eddington and Kepler. K stars are found to be the most promising targets, while the performance drops significantly for stars earlier than G and younger than 2.0 Gyr. Simulations also show that \emph{Eddington} should detect terrestrial planets orbiting solar-age stars in most of the habitable zone for G2 types and all of it for K0 and K5 types.

astro-ph↗

arXiv2003

A large X-ray flare from the Herbig Ae star V892 Tau

We report the XMM-Newton observation of a large X-ray flare from the Herbig Ae star V892 Tau. The apparent low mass companion of V892 Tau, V892 Tau NE, is unresolved by XMM-Newton. Nevertheless there is compelling evidence from combined XMM-Newton and Chandra data that the origin of the flare is the Herbig Ae star V892 Tau. During the flare the X-ray luminosity of V892 Tau increases by a factor of ~15, while the temperature of the plasma increases from kT ~ 1.5 keV to kT ~ 8 keV. From the scaling of the flare event, based on hydrodynamic modeling, we conclude that a 500 G magnetic field is needed in order to confine the plasma. Under the assumptions that a dynamo mechanism is required to generate such a confining magnetic field and that surface convection is a necessary ingredient for a dynamo, our findings provide indirect evidence for the existence of a significant convection zone in the stellar envelope of Herbig Ae stars.

astro-ph↗

arXiv2003

The photospheric abundances of active binaries II. Atmospheric parameters and abundance patterns for 6 single-lined RS CVn systems

Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been derived for 13 chemical species, including several key elements such as Li, Mg, and Ca. Two methods have been used. The effective temperatures, surface gravities and microturbulent velocities were first derived from a fully self-consistent analysis of the spectra, whereby the temperature is determined from the excitation equilibrium of the Fe I lines. The second approach relies on temperatures derived from the (B-V) colour index. These two methods give broadly consistent results for the stars in our sample, suggesting that the neutral iron lines are formed under conditions close to LTE. We discuss the reliability in the context of chromospherically active stars of various colour indices used as temperature indicators, and conclude that the (V-R) and (V-I) colours are likely to be significantly affected by activity processes. Irrespective of the method used, our results indicate that the X-ray active binaries studied are not as metal poor as previously claimed, but are at most mildly iron-depleted relative to the Sun (-0.41<[Fe/H]<+0.11). A significant overabundance of several chemical species is observed (e.g., the alpha-synthezised elements). These abundance patterns are discussed in relation to stellar activity.

astro-ph↗

arXiv2003

Luminosity and Mass Function of the Galactic open cluster NGC 2422

We present UBVRI photometry of the open cluster NGC 2422 (age $\sim 10^8$ yr) down to a limiting magnitude $V\simeq19$. These data are used to derive the Luminosity and Mass Functions and to study the cluster spatial distribution. By considering the color-magnitude diagram data and adopting a representative cluster main sequence, we obtained a list of candidate cluster members based on a photometric criterion. Using a reference field region and an iterative procedure, a correction for contaminating field stars has been derived in order to obtain the Luminosity and the Mass Functions in the $M=0.4-3.5 M_\odot$ range. By fitting the spatial distribution, we infer that a non-negligible number of cluster stars lies outside our investigated region. We have estimated a correction to the Mass Function of the cluster in order to take into account the "missing" cluster stars. The Present Day Mass Function of \ngc2422 can be represented by a power-law of index $α= 3.07 \pm0.08 $ (rms) -- the Salpeter Mass Function in this notation has index $α= 2.35$ -- in the mass range $ 0.9 \leq M/M_\odot\leq 2.5 $. The index $α$ and the total mass of the cluster are very similar to those of the Pleiades.

astro-ph↗

arXiv2003

An XMM-Newton-based X-ray survey of pre-main sequence stellar emission in the L1551 star-forming complex

We present a study of the X-ray sources present in the nearby L1551 star forming region, based on a deep XMM observation complemented with Chandra data for the brightest sources. All the known pre-main sequence stars in the region have been detected, most of them with sufficient statistics to allow a detailed study of the temporal and spectral characteristics of their X-ray emission. Significant temporal (and spectral) variability on both short and long time scales is visible for most of the stars. In particular XZ Tau shows large-amplitude variations on time scales of several hours with large changes in the intervening absorption, suggestive of the X-ray emission being eclipsed by the accretion stream (and thus of the X-ray emission being partly or totally accretion-induced). The coronal metal abundance of the WTTS sources is clustered around Z = 0.2, while the CTTS sources span almost two orders of magnitudes in coronal Z, even though the photospheric abundance of all stars in the L1551 is likely to be very similar. Some individual elements (notably Ne) appear to be systematically enhanced with respect to Fe in the WTTS stars. The significant differences between the spectral and temporal characteristics of the CTTS and WTTS populations suggest that a different emission mechanism is (at least partly) responsible for the X-ray emission of the two types of stars.

astro-ph↗

arXiv2002

The photospheric abundances of active binaries I Detailed analysis of HD 113816 (IS Vir) and HD 119285 (V851 Cen)

The high-resolution optical spectra of the two X-ray active binaries RS CVn stars HD 113816 (IS Vir) and HD 119285 (V851 Cen) are analysed and their Na, Mg, Al, Si, Ca, Sc, Ti, Co and Ni contents determined. The analysis of IS Vir and V851 Cen is performed with three different LTE methods. In the first one, abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software package. The input atmospheric parameters and abundances are iteratively modified until (i) the Fe I abundances exhibit no trend with excitation potential or equivalent width, (ii) Fe I and Fe II average abundances are the same and (iii) Fe and Alpha elements average abundances are consistent with the input values. The second method follows a similar approach, but relies on the B-V and V-I colour indices to determine the temperature. The third method relies on fitting the 6162 A Ca I line wing profiles to derive the surface gravity. The reliability of these methods is investigated in the context of single line RS CVn stars. It is shown that the V-I photometric index gives, on a broader sample of stars, significantly cooler estimates of the effective temperature than the B-V index. All other approaches give results in good agreement with each other. The analysis of IS Vir and V851 Cen results in both cases in their primaries being giant stars of near-solar metallicity. Their parameters as derived with the first method are respectively Teff = 4720 K, log g = 2.65, [Fe/H] = +0.04 and Teff = 4700 K, log g = 3.0 and [Fe/H] = -0.13. In the case of V851 Cen the derived iron content is significantly higher than a previous determination in the literature. Both stars exhibit relative overabundances of several elements (e.g. Ca) with respect to the solar mix.

astro-ph↗