arXiv subjects

N. Mowlavi

Publications and source records attributed to N. Mowlavi.

100 records · Page 6Linked to original sources

arXiv2003

High-Energy sources before INTEGRAL -- INTEGRAL reference catalog --

We describe the INTEGRAL reference catalog which classifies previously known bright X-ray and gamma-ray sources before the launch of INTEGRAL. These sources are, or have been at least once, brighter than ~1 mCrab above 3 keV, and are expected to be detected by INTEGRAL. This catalog is being used in the INTEGRAL Quick Look Analysis to discover new sources or significantly variable sources. We compiled several published X-ray and gamma-ray catalogs, and surveyed recent publications for new sources. Consequently, there are 1122 sources in our INTEGRAL reference catalog. In addition to the source positions, we show an approximate spectral model and expected flux for each source, based on which we derive expected INTEGRAL counting rates. Assuming the default instrument performances and at least ~10^5 sec exposure time for any part of the sky, we expect that INTEGRAL will detect at least ~700 sources below 10 keV and ~400 sources above 20 keV over the mission life.

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arXiv2003

The INTEGRAL Science Data Centre (ISDC)

The INTEGRAL Science Data Centre (ISDC) provides the INTEGRAL data and means to analyse them to the scientific community. The ISDC runs a gamma ray burst alert system that provides the position of gamma ray bursts on the sky within seconds to the community. It operates a quick-look analysis of the data within few hours that detects new and unexpected sources as well as it monitors the instruments. The ISDC processes the data through a standard analysis the results of which are provided to the observers together with their data.

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arXiv2001

Long Period Variable Stars: galactic populations and infrared luminosity calibrations

In this paper HIPPARCOS astrometric and kinematic data are used to calibrate both infrared luminosities and kinematical parameters of Long Period Variable stars (LPVs). Individual absolute K and IRAS 12 and 25 luminosities of 800 LPVs are determined and made available in electronic form. The estimated mean kinematics is analyzed in terms of galactic populations. LPVs are found to belong to galactic populations ranging from the thin disk to the extended disk. An age range and a lower limit of the initial mass is given for stars of each population. A difference of 1.3mag in K for the upper limit of the Asymptotic Giant Branch is found between the disk and old disk galactic populations, confirming its dependence on the mass in the main sequence. LPVs with a thin envelope are distinguished using the estimated mean IRAS luminosities. The level of attraction (in the classification sense) of each group for the usual classifying parameters of LPVs (variability and spectral types) is examined.

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arXiv2000

Chemical abundances in seven red giants of NGC 2360 and NGC 2447

Chemical abundances of about fifteen elements from oxygen to europium are measured in seven red giants of the two open clusters NGC 2360 and NGC 2447. The effective temperatures of the giants are determined spectroscopically by taking advantage of their known masses (~2 Mo in NGC 2360 and ~3 Mo in NGC 2447) and bolometric magnitudes. The average iron abundances we obtain for the two clusters are [Fe/H]=0.07 for NGC 2360 and [Fe/H]=0.03 for NGC 2447. Evolutionary stellar model calculations are performed in the mass range 1-4 Mo in order to analyze the surface Na and O abundances predicted after the first dredge-up. The sodium abundance shows a well defined correlation with stellar mass in the 2-3 Mo range. The agreement between our Na abundance determinations in NGC 2360 and our model predictions at 2 Mo is very good. In contrast, the overabundance in one of the three stars in NGC 2447 exceeds that predicted at 3 Mo by ~0.08 dex, which is significant compared to the observational error bars. The effects of core overshooting, convection prescription, metallicity and nuclear reaction rates on the Na surface predictions of our models are investigated. An oxygen deficiency relative to iron by 0.2 dex is measured in our stars, in disagreement with our model predictions. Assuming that the Sun is 0.1-0.3 dex enriched in oxygen relative to neighbor stars could explain the discrepancy.

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arXiv1999

Some selected comments on cosmic radioactivities

Radionuclides with half-lives ranging from some years to billions of years presumably synthesized outside of the solar system are now recorded in `live' or `fossil' form in various types of materials, like meteorites or the galactic cosmic rays. They bring specific astrophysical messages the deciphering of which is briefly reviewed here, with special emphasis on the contribution of Jerry Wasserburg and his collaborators to this exciting field of research. A brief mention is also made to $γ$-ray line astrophysics. It provides an interesting information on live short-lived radionuclides in the present interstellar medium, and thus complements in a very important way the study of extinct radionuclides in meteorites.

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arXiv1999

On the third dredge-up phenomenon in asymptotic giant branch stars

The third dredge-up phenomenon in asymptotic giant branch (AGB) stars is analyzed through evolutionary model calculations of a \mass{3}, solar metallicity star. The Schwarzschild criterion is used to test the stability of a given layer against convection, and the calculations are performed either with or without extra-mixing below the convective envelope. Based on these calculations, several questions are addressed regarding the occurrence of the third dredge-up in AGB star models, the laws governing that phenomenon, and some of its implications on the structural and chemical evolution of those stars.

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arXiv1998

Grids of stellar models. VIII. From 0.4 to 1.0 Msun at Z=0.020 and Z=0.001, with the MHD equation of state

We present stellar evolutionary models covering the mass range from 0.4 to 1 Msun calculated for metallicities Z=0.020 and 0.001 with the MHD equation of state (Hummer & Mihalas, 1988; Mihalas et al. 1988; Däppen et al. 1988). A parallel calculation using the OPAL (Rogers et al. 1996) equation of state has been made to demonstrate the adequacy of the MHD equation of state in the range of 1.0 to 0.8 Msun (the lower end of the OPAL tables). Below, down to 0.4 Msun, we have justified the use of the MHD equation of state by theoretical arguments and the findings of Chabrier & Baraffe (1997). We use the radiative opacities by Iglesias & Rogers (1996), completed with the atomic and molecular opacities by Alexander & Fergusson (1994). We follow the evolution from the Hayashi fully convective configuration up to the red giant tip for the most massive stars, and up to an age of 20 Gyr for the less massive ones. We compare our solar-metallicity models with recent models computed by other groups and with observations. The present stellar models complete the set of grids computed with the same up-to-date input physics by the Geneva group [Z=0.020 and 0.001, Schaller et al. (1992), Bernasconi (1996), and Charbonnel et al. (1996); Z=0.008, Schaerer et al. (1992); Z=0.004, Charbonnel et al. (1993); Z=0.040, Schaerer et al. (1993); Z=0.10, Mowlavi et al. (1998); enhanced mass loss rate evolutionary tracks, Meynet et al. (1994)].

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arXiv1998

On some properties of very metal-rich stars

An analysis of some properties of stellar models as a function of metallicity Z (and helium content Y) is presented, with special attention to those stars with metallicities higher than twice or three times solar. It is shown that the stellar properties as a function of Z are mainly determined by the effects of the opacities at sub-solar metallicities, and by the effects of the mean molecular weight and stellar mass loss at higher metallicities. As a result, very metal-rich stars (Z >~0.05) exhibit properties that deviate from what is expected from the known characteristics at lower metallicities. In particular, they are more luminous and hotter than those at Z <~ 0.05 due to the effect of the mean molecular weight. They have main sequence lifetimes much shorter (60 % shorter at Z=0.1 than at Z=0.02) than those at solar metallicity due to their lower initial hydrogen content. Finally, the high mass loss rates at high metallicities affect significantly the population synthesis of massive stars in very metal-rich regions. An analysis of expected Wolf-Rayet and supernovae populations in such conditions is briefly presented.

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arXiv1996

Fluorine production in intermediate-mass stars

The 19F production during the first dozen thermal pulses of AGB stars with (M=3,Z=0.02), (M=6,Z=0.02) and (M=3,Z=0.001) is investigated on grounds of detailed stellar models and of revised rates for 15N(a,g)19F and 18O(a,g)22Ne. These calculations confirm an early expectation that 19F {\it is} produced in AGB thermal pulses. They also enlarge substantially these previous results by showing that the variations of the level of 19F production during the evolution is very sensitive to the maximum temperature reached at the base of the pulse. These variations are analyzed in detail, and are shown to result from a subtle balance between different nuclear effects (mainly 19F production or destruction in a pulse, and 15N synthesis during the interpulse), possibly super-imposed on dilution effects in more or less extended pulse convective tongues. Our calculations, as most others, do not predict the third dredge-up self- consistently. When parametrized, it appears that our models of intermediate-mass AGB stars are able to account only for the lowest 19F overabundances observed in solar-metallicity MS, S and C stars. That conclusion is expected to hold true for low-mass stars when F production results from secondary 13C only. Massive AGB stars, on the other hand, are not expected to build up large surface F abundanc- es. Therefore, the large F overabundance reported for the super Li-rich star WZ Cas (where HBB is supposed to be operating) remains unexplained so far. Our results for the (M=3,Z=0.001) star indicate that F surface overabundances can also be expected in low-metallicity stars provided that third dredge-ups occur after the early cool pulses. The relative increase in the surface 19F/12C ratio is, however, lower in the low-metallicity than in the solar-metallicity star. No observations are

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arXiv1995

Non-Explosive Hydrogen Burning: Where Do We Stand?

The impact of nuclear physics uncertaintites on the abundances predicted to emerge from the cold CNO, NeNa and MgAl modes of hydrostatic hydrogen burning is discussed in the framework of a simple parametric model. In addition of being able to mimic qualitatively detailed stellar model predictions, these parametric calculations have the virtue of isolating in a crystal-clear way abundance uncertainties of purely nuclear physics origin.

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