arXiv subjects

R. Pallavicini

Publications and source records attributed to R. Pallavicini.

25 records · Page 2Linked to original sources

arXiv2001

Membership, lithium, and metallicity in the young open clusters IC 2602 and IC 2391: enlarging the sample

We present lithium abundances for ~50 X-ray selected candidate members of the 30-50 Myr old open clusters IC 2602 and IC 2391. These data enlarge and extend to cooler temperatures previous Li surveys of these clusters by Stauffer et al. (1989) and Randich et al. (1997). We also give for the first time an estimate of the metallicity of the two clusters which turns out to be close to solar. Radial velocity measurements together with Halpha chromospheric emission and the presence/absence of other spectroscopic features are used to ascertain the membership status for the sample stars not yet confirmed as cluster members; rotational velocities have also been determined for all sample stars. Stars more massive than ~1 Mo in both clusters show no sign of significant Li depletion, while lower mass stars are all lithium depleted, with the amount of Li depletion increasing to cooler temperatures. We confirm that the late G and early K stars in IC 2602 present a star-to-star scatter in Li abundances similar to, but not as large as the one in the Pleiades. A scatter is also seen among late-K and M dwarfs. Unlike in the Pleiades and Alpha Per clusters, the scatter among early-K stars in IC 2602 shows only marginal correlation with rotation. Our data suggest that the drop-off of lithium towards lower masses may start at an earlier color in IC 2391 than in IC 2602, but larger cluster samples are needed to confirm this result. In addition, whereas G and early K stars in the two clusters are, on average, more Li rich than their counterparts in the Pleiades, a fraction of the coolest stars, in particular in IC 2391, are as depleted as as the lowest-Li Pleiades stars of the same mass. If they continue depleting Li on their way to the main sequence, they are expected to be more Li depleted than the Pleiades at the age of the latter cluster.

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arXiv2000

A ROSAT HRI study of the open cluster NGC 3532

NGC 3532 is a very rich southern open cluster of age ~200-350 Myr; it is therefore a good candidate to investigate the X-ray activity-age-rotation relationship at ages intermediate between the Pleiades and the Hyades, where, to our knowledge, X-rays studies exist for only one cluster (NGC 6475). We have performed an X-ray study of NGC 3532 using HRI observations retrieved from the ROSAT archive. The observations have a limiting sensitivity $L_{\mathrm x} \sim 4 \times 10^{28}$ erg sec$^{-1}$ in the center of the field. We detected ~50 X-ray sources above a $4 σ$ threshold, half of which have a known optical counterpart within 10 arcsec; 15 of the X-ray sources have at least one cluster member as optical counterpart. A comparison of NGC 3532 with the nearly coeval cluster NGC 6475 indicates that the former cluster is considerably X-ray underluminous with respect to NGC 6475. However, because of the existence of possible selection effects, additional X-ray and optical observations are needed before definitively concluding that the X-ray properties of NGC 3532 and NGC 6475 are significantly different.

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arXiv2000

Evolution of lithium in solar-type stars: clues from intermediate age clusters

We present Li abundances for 14 solar-type stars in the intermediate age (~ 2 Gyr) clusters IC 4651 and NGC 3680. The log n(Li) vs. effective temperature distributions are compared with those of the similar age cluster NGC 752, of the younger Hyades (600 Myr) and of the older M 67 (4.5 Gyr) and NGC 188 (6-7 Gyr) clusters. Neither IC 4651 nor NGC 3680 show the dispersion in Li which is observed in M 67. The 2 Gyr clusters have very similar Li vs. effective temperature distributions; in addition, stars in the upper envelope of the M 67 distribution have the same Li content as stars in the 2 Gyr clusters, suggesting that either they have not suffered any significant depletion between 2 and 4.5 Gyr or they had a much slower Li depletion. Mechanisms that lead to Li depletion on the main sequence are discussed in the light of these observations. None of the existing models seem to reproduce well the observed features.

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arXiv1999

The active binary star IIPeg with BeppoSAX

IIPeg is an ideal target to study stellar activity and flares, since intense and long lasting flares have been frequently detected from this system at all wavelengths. We report here about a BeppoSAX observation of IIPeg. We followed the system for ~19 hours on December 5 and 6 1997 with BeppoSAX and the X-ray light curve resembles the typical behavior of a decay phase of a long-lasting flare. The spectral analysis shows that the IIPeg X-ray spectrum is described by a two-temperature components, with the two dominant temperatures centered in the range of 9-11 and 24-26 MK. The derived coronal metal abundance is low Z~0.2 Zo compared to recent determinations of the photospheric abundance (Z~0.6 Zo). Some possible explanations for this phenomenology are reviewed. As for most other stellar coronal sources observed with BeppoSAX, we find that in order to fit the BeppoSAX spectra an interstellar column density about a factor ten higher than previously determined is required.

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arXiv1999

A BeppoSAX Observation of HD9770: a Visual Triple System Containing a Recently Discovered Short-Period Eclipsing Binary

We have studied the coronal X-ray emission of the recently discovered short-period eclipsing binary HD 9770 with the BeppoSAX satellite. The data from the Low Energy and Medium Energy Concentrator Spectrometers (LECS & MECS) onboard BeppoSAX allow studying the spectrum of this star from 0.1 to 8 keV, confirming that this is a very active coronal source, with strong flaring activity. The X-ray emission most likely originates from the eclipsing binary itself, rather than from the other visual component of the system. The X-ray light curves could be modulated with the orbital period of the eclipsing binary, with a hint for a different orbital modulation of the cooler and hotter plasma. The X-ray spectrum is characterized by hot plasma, with the Fe K complex at 6.7 keV clearly detected in the MECS spectrum, and it is well fitted by a 2-temperature optically thin plasma model with low metal abundances ($\sim 0.3 Z_\odot$). These results are in line with those found for many other active stars. As expected, during the flare the X-ray emission is dominated by hotter plasma with a temperature $> 4$ keV. There is an indication that the metal abundance may be somewhat higher during the flare.

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arXiv1997

A SAX/LECS X-ray observation of the active binary Capella

We present a SAX/LECS X-ray observation of the active binary Capella, the first coronal source observed in the SAX Guest Investigator program. The analysis of this observation, performed using the mekal plasma emission code, shows that the LECS spectrum is well fit by a two-component optically-thin plasma model. A differential emission measure (DEM) obtained by direct inversion of the X-ray spectrum shows no additional features in addition to the double-peaked structure implied by the direct two-temperature analysis. Such a simple temperature stratification is however not compatible with the EUVE emission from the same object, which is well represented by a more complex DEM, with a power-law-like tail toward the low temperatures. At the same time, the EUVE-derived DEM predicts well the softer part of the Capella LECS spectrum, but it fails to correctly reproduce the higher energy part of the Capella LECS spectrum. Possible causes for this discrepancy are discussed. The coronal metallicity derived from the SAX observation is compatible both with the EUVE-derived metallicity as well as with the photospheric metallicity of Capella, thus showing no evidence for coronal under-abundances.

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arXiv1996

Coronal X-Ray Emission of HD35850: the ASCA view

We present the analysis of the X-ray data of the young active star HD35850 obtained with ASCA and ROSAT. Our main goal was to see if there is a difference in the elemental abundances of active stars between young and more evolved objects. A two temperature plasma with subsolar abundances, of the order of Z = 0.15 - 0.3, is required to fit the SIS spectra. Similar results are obtained from a ROSAT PSPC observation. Metal abundances of 0.2 - 0.4 the solar value are required to fit both the ASCA and ROSAT data together. From a simultaneous SIS0+SIS1 spectral fit, with 2T plasma models and abundances free to vary in non-solar proportions, we find that, besides N, O and Ne for which we find very low values, all other elements have values relative to solar abundances around 0.2-0.3. These subsolar abundances are in line with those typically observed in more evolved, active stars like RS CVn and Algol-type binaries. The two temperature values required to fit the ASCA SIS spectra are about 0.5 and 1.0 keV. These temperatures, especially the higher one, are lower with respect to the values found for the RS CVn and Algol binaries or for the young star AB Dor, but higher than other single G/K stars. All our data show that this single, late F-type star is actually a very active source, indirectly confirming that this fast rotating star is probably a young object. In the simultaneous fit of the ASCA+ROSAT data, a third temperature is required. However this is not just an addition of a softer component, but is more a redistribution of the dominant temperatures. Indeed, the range spanned by the three temperatures, from 5 to 15 million degrees, is not very large.

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