arXiv subjects

G. Micela

Publications and source records attributed to G. Micela.

At least 343 records · Page 19Linked to original sources

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.

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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 $α=2.35$.

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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.

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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.

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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.

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arXiv2003

X-ray rotational modulation of a supersaturated star in IC 2391

We present evidence of X-ray rotational modulation on VXR45, a young fast rotator star, member of IC 2391. It is a dG9 spectral type star whose rotational period and X-ray luminosity make it a supersaturated star. Our X-ray observation, made with EPIC/PN on XMM-Newton, covers about two photometric rotational periods. The detection of rotational modulation implies the presence of structural inhomogeneities. Possible interpretations are presented and discussed.

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arXiv2003

Three years in the coronal life of AB Dor. I. Plasma emission measure distributions and abundances at different activity levels

The young active star AB Dor (K1 IV-V) has been observed 16 times in the last three years with the XMM-Newton and Chandra observatories, totalling 650 ks of high-resolution X-ray spectra. The XMM/RGS observations with the highest and lowest average emission levels have been selected to study the coronal properties of AB Dor in two different activity levels. We compare the results based on the XMM data with those obtained from a higher resolution Chandra/HETG spectrum, using the same line-based analysis technique. We have reconstructed the plasma Emission Measure Distribution vs. temperature (EMD) in the range log T(K) ~ 6.1-7.6, and we have determined the coronal abundances of AB Dor, obtaining consistent results between the two instruments.The overall shape of the EMD is also consistent with the one previously inferred from EUVE data. The EMD shows a steep increase up to the peak at log T (K) ~ 6.9 and a substantial amount of plasma in the range log T (K) ~ 6.9-7.3. The coronal abundances show a clear trend of increasing depletion with respect to solar photospheric values, for elements with increasing First Ionization Potential (FIP), down to the Fe value ([Fe/H]=-0.57), followed by a more gradual recovery of the photospheric values for elements with higher FIP. He-like triplets and Fe XXI and Fe XXII lines ratios indicate electron densities log ne~10.8 cm^-3 at log T(K) ~ 6.3 and log ne ~ 12.5 at log T(K) ~ 7, implying plasma pressures steeply increasing with temperature. These results are interpreted in the framework of a corona composed of different families of magnetic loop structures, shorter than the stellar radius and in isobaric conditions, having pressures increasing with the maximum plasma temperature, and which occupy a small fraction (f ~ 10^-4 - 10^-6) of the stellar surface.

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arXiv2003

Late B-type stars and their candidate companions resolved with Chandra

We present the first results from a series of Chandra observations carried out with the aim to examine the origin of X-ray emission in main-sequence late B-type stars. We use high spatial resolution X-ray observations to enlighten the prevalent speculation that previously unknown late-type or low-mass companion stars are the sites of the X-ray emission, instead of the B-type primaries. Here we present the results for HD1685, HD113703, HD123445, HD133880, and HD169978. Adaptive optics observations have recently revealed at least one faint object near each of these B-type stars (at separation of 1-6"). Four of the new infrared objects show infrared colors and magnitudes typical for low-mass pre-main sequence stars, and are likely true companions to the 10-50 Myr old B-type stars. These multiple systems are now resolved for the first time in X-ray light.

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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.

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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.

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arXiv2003

Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR of varying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so to minimize systematic differences in the results. We study trends of L_X and L_X/L_bol as a function of stellar mass and association age. For low mass stars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr and a subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ~10^7 years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of L_X/L_bol with time.

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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.

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arXiv2002

Observational clues for a role of circumstellar accretion in PMS X-ray activity

We revisit the published analyses of ROSAT X-ray observations of the star forming regions NGC 2264 and Chamaeleon I (~3 and ~5 Myr old respectively) in the light of newly published optical data. At odds with previous results on Chamaeleon I members, we find that low mass stars in both regions have near-saturated emission levels. Similarly to what previously found in the Orion Nebula Cluster, Weak Line T-Tauri Stars in NGC 2264 and in the Chamaeleon I cloud have higher X-ray activity levels respect to Classical T Tauri Stars, arguing in favor of a role of the disk and/or accretion in determining X-ray emission.

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arXiv2002

Chandra X-ray Observation of the Orion Nebula Cluster. I Detection, Identification and Determination of X-ray Luminosities

In this first of two companion papers on the Orion Nebula Cluster (ONC), we present our analysis of a 63 Ksec Chandra HRC-I observation that yielded 742 X-ray detections within the 30'x30' field of view. To facilitate our interpretation of the X-ray image, here we collect a multi-wavelength catalog of nearly 2900 known objects in the region by combining 17 different catalogs from the recent literature. We define two reference groups: an infrared sample, containing all objects detected in the K band, and an optical sample comprising low extinction, well characterized ONC members. We show for both samples that field object contamination is generally low. Our X-ray sources are primarily low mass ONC members. The detection rate for optical sample stars increases monotonically with stellar mass from zero at the brown dwarf limit to ~100% for the most massive stars but shows a pronounced dip between 2 and 10 solar masses. We determine L_X and L_X/L_{bol} for all stars in our optical sample and utilize this information in our companion paper to study correlations between X-ray activity and other stellar parameters.

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arXiv2002

Chandra X-ray Observation of the Orion Nebula Cluster. II Relationship between X-ray activity indicators and stellar parameters

Using the results of our first paper on the Chandra HRC observation of the Orion Nebula Cluster (ONC), here we explore the relation between the coronal activity of its 1-Myr-old pre-main sequence population and stellar parameters. We find that median X-ray luminosities of low mass stars (M/M_sun < 3) increase with increasing mass and decreasing stellar age. Brown dwarfs (0.03 < M/M_sun < 0.08) follow the same trend with mass. From M~0.1 to M~0.5M_sun, median L_X/L_bol values increase by about half an order of magnitude and then remain constant at ~10^-3.5 for the mass range from 0.5 to 3.0 M/M_sun. In these same two mass ranges, L_X/L_bol remains roughly constant with age, until it drops by more than two orders of magnitudes at the epoch when ~2-4M_sun stars are expected to become fully radiative. We find a dependence of L_X and L_X/L_bol on circumstellar accretion indicators and suggest three possible hypotheses for its origin. In spite of improved X-ray and rotational data, correlations between activity indicators and rotation remain elusive for these stars, possibly indicating that stars for which rotational periods have been measured have reached some saturation level. Our study of X-ray activity vs. stellar mass leads us to propose that the few HRC X-ray sources not associated with any optical/infrared counterpart trace a yet to be discovered stellar population of deeply embedded, relatively massive ONC members.

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arXiv2002

The EUVE point of view of AD Leo

All the Extreme Ultraviolet Explorer (EUVE) observations of AD Leo, totalling 1.1 Ms of exposure time, have been employed to analyze the corona of this single M dwarf. The light curves show a well defined quiescent stage, and a distribution of amplitude of variability following a power law with a ~-2.4 index. The flaring behavior exhibits much similarity with other M active stars like FK Aqr or YY Gem, and flares behave differently from late type active giants and subgiants. The Emission Measure Distribution (EMD) of the summed spectrum, as well as that of quiescent and flaring stages, were obtained using a line-based method. The average EMD is dominated by material at log T(K)~6.9, with a second peak around log T(K)~6.3, and a large increase in the amount of material with log T(K)>~7.1 during flares, material almost absent during quiescence. The results are interpreted as the combination of three families of loops with maximum temperatures at log T(K)~6.3, ~6.9 and somewhere beyond log T(K)>~7.1. A value of the abundance of [Ne/Fe]=1.05+-0.08 was measured at log T(K)~5.9. No significative increment of Neon abundance was detected between quiescence and flaring states.

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arXiv2001

Discovery of X-ray emission from the proto-stellar jet L1551 IRS5 (HH 154)

We have for the first time detected X-ray emission associated with a proto-stellar jet, on the jet emanating from L1551 IRS5. The IRS5 proto-star is hidden beyond a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock ``working surface'', i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is moderate, at LX ~ 3 times 10^29 erg/s, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with T ~ 0.5 MK, fully compatible with the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.

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arXiv2001

The discovery of X-ray emission from the protostellar jet L1551 IRS5 (HH 154)

We have for the first time detected X-ray emission associated with a protostellar jet, on the jet emanating from the L1551 IRS5 protostar. The IRS5 protostar is hidden behind a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock ``working surface'', i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is moderate, at $L_{\rm X} \simeq 3 \times 10^{29}$ erg/s, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with a temperature of $\simeq 0.5$ MK, fully in line with the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.

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