arXiv subjects

Mohammad Taghi Mirtorabi

Publications and source records attributed to Mohammad Taghi Mirtorabi.

14 recordsLinked to original sources

arXiv2026

The B-Index as a Diagnostic of Cool Stars: Assessing Metallicity Dependence of TiO Absorption in the Visible Spectrum

Molecular absorption features constitute essential diagnostics of late-type stellar atmospheres, with titanium oxide (TiO) bands serving as sensitive tracers of effective temperature and magnetic activity. While near-infrared TiO indices are well established, visible-band diagnostics provide complementary constraints for stellar classification and parameter estimation. This study investigates the metallicity dependence of the B-index, a TiO absorption measure centered at 567 nm, through synthetic spectra generated with ATLAS9 and high-resolution observations from the HARPS spectrograph. The synthetic grid spans effective temperatures between 3500-4000 K and metallicities from [Fe/H] =-4 to +0.2, while the observational sample comprises 23 MK spectral types stars with effective temperatures between 3500-4000 K. Both theoretical and empirical analyses demonstrate that the B-index exhibits negligible sensitivity to stellar metallicity, with only marginal correlations detected. Therefore, there is no requirement to include this feature in the calculation of the B-index. The B-index's low sensitivity to metallicity reinforces its utility as an effective tool for spectral classification, stellar atmosphere modeling, and the study of magnetically active stars.

astro-ph.SR↗

arXiv2026

Titanium Oxide Absorption as a Proxy to Detect Long term Variation and Activity Cycle in Proxima Centauri

Stellar activity cycles on magnetically active stars can be estimated by molecular absorption bands. We have previously introduced a molecular index which compares absorptional line strength of the TiO567nm with its nearby continuum has previously been introduced. In this work we use this indicator to evaluation long-term activity variations for Proxima Centauri star, using spectroscopic data from HARPS. The results indicate periodicity with an activity period of 2873(+47.4-53.9) days, which is similar to the previous measurements from other indicators.

astro-ph.SR↗

arXiv2023

The formation and evolution of Andromeda IX

Local Group (LG), the nearest and most complete galactic environment, provides valuable information on the formation and evolution of the Universe. Studying galaxies of different sizes, morphologies, and ages can provide this information. For this purpose, we chose the And\,IX dSph galaxy, which is one of the observational targets of the Isaac Newton Telescope (INT) survey. A total of 50 long-period variables (LPVs) were found in And\,IX in two filters, Sloan $i'$ and Harris $V$ at a half-light radius of 2.5 arcmin. The And\,IX's star formation history (SFH) was constructed with a maximum star formation rate (SFR) of about $0.00082\pm0.00031$ M$_\odot$ yr$^{-1}$, using LPVs as a tracer. The total mass return rate of LPVs was calculated based on the spectral energy distribution (SED) of about $2.4\times10^{-4}$ M$_\odot$ yr$^{-1}$. The distance modulus of $24.56_{-0.15}^{+0.05}$ mag was estimated based on the tip of the red giant branch (TRGB).

astro-ph.GA↗

arXiv2023

The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. VI. The Star Formation History and Dust Production in Andromeda IX

We present a photometric study of the resolved stellar populations in And IX, the closest satellite to the M31, a metal-poor and low-mass dwarf spheroidal galaxy. We estimate a distance modulus of $24.56_{-0.15}^{+0.05}$ mag based on the tip of the red giant branch (TRGB). By probing the variability of asymptotic giant branch stars (AGB), we study the star formation history of And IX. We identified 50 long period variables (LPVs) in And IX using the Isaac Newton Telescope (INT) in two filters, Sloan $i'$ and Harris $V$. In this study, we selected LPVs within two half-light radii with amplitudes in the range of 0.2-2.20 mag. It is found that the peak of star formation reaches $\sim$ $8.2\pm3.1\times10^{-4}$ M_sun yr$^{-1}$ at $\approx 6$ Gyr ago. Our findings suggest an outside-in galaxy formation scenario for And IX with a quenching occurring $3.65_{-1.52}^{+0.13}$ Gyr ago with the SFR in the order of $2.0\times10^{-4}$ M_sun yr$^{-1}$ at redshift < $0.5$. We calculate the total stellar mass by integrating the star formation rate (SFR) within two half-light radii $\sim$ $3.0\times10^5$ M_sun. By employing the spectral energy distribution (SED) fitting for observed LPVs in And IX, we evaluate the mass-loss rate in the range of $10^{-7}$ $\leq$ $\dot{M}$ $\leq$ $10^{-5}$ M_sun yr$^{-1}$. Finally, we show that the total mass deposition to the interstellar medium (ISM) is $\sim$ $2.4\times10^{-4}$ M_sun yr$^{-1}$ from the C- and O-rich type of dust-enshrouded LPVs. The ratio of the total mass returned to the ISM by LPVs to the total stellar mass is $\sim 8.0\times10^{-10}$ yr$^{-1}$, and so at this rate, it would take $\sim$ 1 Gyr to reproduce this galaxy

astro-ph.GA↗

arXiv2021

INT monitoring survey of Local Group dwarf galaxies: star formation history and chemical enrichment

The Local Group (LG) hosts many dwarf galaxies with diverse physical characteristics in terms of morphology, mass, star formation, and metallicity. To this end, LG can offer a unique site to tackle questions about the formation and evolution of galaxies by providing detailed information. While large telescopes are often the first choices for such studies, small telescope surveys that perform dedicated observations are still important, particularly in studying bright objects in the nearby universe. In this regard, we conducted a nine epoch survey of 55 dwarf galaxies called the Local Group dwarf galaxies survey using the 2.5m Isaac Newton Telescope (INT) in La Palma to identify Long-Period Variable (LPV) stars, namely Asymptotic Giant Branch (AGB) and Red Super Giant (RSG) stars. AGB stars formed at different times and studying their radial distribution and mass-loss rate can shed light on the structure formation in galaxies. To further investigate the evolutionary path of these galaxies, we construct their star formation history (SFH) using the LPV stars, which are at the final stages of their evolution and therefore experience brightness fluctuations on the timescales between hundred to thousand days. In this paper, we present some of the results of the Local Group dwarf galaxies survey.

astro-ph.GA↗

arXiv2019

Chameleon Mechanism in Inhomogeneous Astrophysical Objects

Observational evidence implying the accelerated expansion of the universe has been the motivation to develop various classes of modified gravity theories. One of them uses the so-called "screening mechanism", which is successful in reproducing the observed gravitational behavior in large scales as well as being in agreement with tests of general relativity in the solar system. In this work, we investigate an example of scalar-tensor theories with screening mechanism, namely the profile of a Chameleon field around inhomogeneous astrophysical objects. According to [Khoury and Weltman(2004)], one can define two kinds of approaches applicable to the thin shell and thick shell regimes, that allow for a solution to the Chameleon equation of motion. For sufficiently large objects, the scalar field can be assumed to propagate from a thin shell of the object instead of the whole body, which simplifies the problem. On the other hand, this solution is not practical in small objects. We find that in inhomogeneous objects this is not trivial and at least one more factor, which turns out to be the density, can change the way of approaching this problem.

gr-qc↗

arXiv2017

A survey of TiOλ567 nm absorption in solar-type stars

Molecular absorption bands are estimators of stellar activity and spot cycles on magnetically active stars. We have previously introduced a new color index that compare absorption strength of the titanium oxide (TiO) at 567 nm with nearby continuum. In this paper we implement this index to measure long-term activity variations and the statistical properties of the index in a sample of 302 solar-type stars from the HARPS planet search program. The results indicate a pattern of change in star's activity, covers a range of periods from 2 years up to 17 years.

astro-ph.SR↗

arXiv2016

A New Titanium Oxide Index in Visual Band

We introduce a new color index consisting of two spectral band width to measure TiO absorption band strength centered at 567 nm. Based on the most up-to-date line list for TiO we regenerate a grid of synthesized spectra and investigate the temperature sensitivity of the index. The new index behave similar to older Wing TiO-index where itdecreas emonotonically from coolest atmosphere with Teff =2800 up to Teff =4000 where the TiO molecules disassociate. To further examine the feasibility of the new index we reproduce the calibration using a list of observed high resolution spectra and found similar results. This index extend TiO absorption band capability to measure effective temperature of late K to M stars to visual spectrum where it is more accessible to small telescopes for long term dedicated observation.

astro-ph.SR↗

arXiv2014

The UK Infrared Telescope M33 monitoring project. IV. Variable red giant stars across the galactic disc

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. In this fourth paper of the series, we present a search for variable red giant stars in an almost square degree region comprising most of the galaxy's disc, carried out with the WFCAM instrument in the K band. These data, taken during the period 2005--2007, were complemented by J- and H-band images. Photometry was obtained for 403 734 stars in this region; of these, 4643 stars were found to be variable, most of which are Asymptotic Giant Branch (AGB) stars. The variable stars are concentrated towards the centre of M33, more so than low-mass, less-evolved red giants. Our data were matched to optical catalogues of variable stars and carbon stars and to mid-infrared photometry from the Spitzer Space Telescope. Most dusty AGB stars had not been previously identified in optical variability surveys, and our survey is also more complete for these types of stars than the Spitzer survey. The photometric catalogue is made publicly available at the Centre de Donnees astronomiques de Strasbourg.

astro-ph.SR↗

arXiv2014

Local stability criterion for self-gravitating disks in modified gravity

We study local stability of self-gravitating fluid and stellar disk in the context of modified gravity theories which predict a Yukawa-like term in the gravitational potential of a point mass. We investigate the effect of such a Yukawa-like term on the dynamics of self-gravitating disks. More specifically, we investigate the consequences of the presence of this term for the local stability of the self-gravitating disks. In fact, we derive a generalized version of Toomre's local stability criterion for diferentially rotating disks. This criterion is complicated than the original one in the sense that it depends on the physical properties of the disk. In the case of MOdified Gravity theory (MOG), we use the current confirmed values for the free parameters of this theory, to write the generalized Toomre's criterion in a more familiar way comparable with the Toomre's criterion. This generalized Toomre's criterion may be used to study the global stability of stellar and fluid disks using computer simulations.

astro-ph.GA↗

arXiv2013

The UK Infrared Telescope M33 monitoring project. III. Feedback from dusty stellar winds in the central square kiloparsec

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. In this third paper of the series, we measure the dust production and rates of mass loss by the pulsating Asymptotic Giant Branch (AGB) stars and red supergiants. To this aim, we combined our time-averaged near-IR photometry with the multi-epoch mid-IR photometry obtained with the Spitzer Space Telescope. The mass-loss rates are seen to increase with increasing strength of pulsation and with increasing bolometric luminosity. Low-mass stars lose most of their mass through stellar winds, but even super-AGB stars and red supergiants lose $\sim40$% of their mass via a dusty stellar wind. More than three-quarters of the dust return is oxygenous. We construct a 2-D map of the mass-return rate, showing a radial decline but also local enhancements due to agglomerations of massive stars. We estimate a total mass-loss rate of 0.004--0.005 M$_\odot$ yr$^{-1}$ kpc$^{-2}$, increasing to $\sim0.006$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$ when accounting for eruptive mass loss (e.g., supernovæ); comparing this to the current star formation rate of $\sim0.03$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$ we conclude that star formation in the central region of M\,33 can only be sustained if gas is accreted from further out in the disc or from circum-galactic regions.

astro-ph.SR↗

arXiv2011

The UK Infrared Telescope M33 monitoring project. II. The star formation history in the central square kiloparsec

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. In this second paper of the series, we construct the birth mass function and hence derive the star formation history. The star formation rate has varied between ~0.002 and 0.007 M$_\odot$ yr^-1 kpc^-2. We give evidence of two epochs of a star formation rate enhanced by a factor of a few -- one that happened \geq 6 Gyr ago and produced \geq 80% of the total mass in stars, and one around 250 Myr ago that lasted ~ 200 Myr and formed \leq 6% of the mass in stars. We construct radial and azimuthal distributions in the image plane and in the galaxy plane for populations associated with old first-ascent red giant branch (RGB) stars, intermediate-age Asymptotic Giant Branch (AGB) stars and young (massive) blue and red supergiants. We find that the RGB stars follow a spheroidal distribution, while younger stars follow a flat-disc distribution. The intermediate-age population displays signs of a pseudo-bulge or possibly a bar. The inner spiral arm pattern as recorded in mid-19^th-century drawings is confirmed. We interpret our findings as evidence for an old, pressure-supported component and a younger disc formed 6 Gyr ago, with an accretion event occurring 250 Myr ago giving rise to the compact nucleus in M33. Our study provides support for recent Padova stellar evolution models except that super-AGB stars likely reach low temperatures and thus high mass-loss rates, supporting the super-AGB nature of the progenitors of dust-enshrouded supernovae such as SN2008S.

astro-ph.SR↗

arXiv2011

Infrared Survey of Pulsating Giant Stars in the Spiral Galaxy M33: Dust Production, Star Formation History, and Galactic Structure

We introduce a near-IR monitoring campaign of the Local Group spiral galaxy M33, carried out with the UK IR Telescope (UKIRT). The pulsating giant stars are identified and their distributions are used to derive the star formation rate as a function of age. We here present the star formation history for the central square kiloparsec. These stars are also important dust factories; we measure their dust production rates from a combination of our data with Spitzer Space Telescope mid-IR photometry.

astro-ph.GA↗

arXiv2010

The UK Infrared Telescope M33 monitoring project. I. Variable red giant stars in the central square kiloparsec

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. The most extensive dataset was obtained in the K-band with the UIST instrument for the central 4'x 4' (1 square kpc) - this contains the nuclear star cluster and inner disc. These data, taken during the period 2003-2007, were complemented by J- and H-band images. Photometry was obtained for 18,398 stars in this region; of these, 812 stars were found to be variable, most of which are Asymptotic Giant Branch (AGB) stars. Our data were matched to optical catalogues of variable stars and carbon stars, and to mid-infrared photometry from the Spitzer Space Telescope. In this first of a series of papers, we present the methodology of the variability survey and the photometric catalogue - which is made publicly available at the Centre de Donnees astronomiques de Strasbourg (CDS) - and discuss the properties of the variable stars. Most dusty AGB stars had not been previously identified in optical variability surveys, and our survey is also more complete for these types of stars than the Spitzer survey.

astro-ph.SR↗