Models For Main Sequence Stars Research Articles

WISE J072003.20-084651.2B is a Massive T Dwarf∗ †

Abstract We present individual dynamical masses for the nearby M9.5+T5.5 binary WISE J072003.20−084651.2AB, a.k.a. Scholz’s star. Combining high-precision Canada–France–Hawaii Telescope/WIRCam photocenter astrometry and Keck adaptive optics resolved imaging, we measure the first high-quality parallactic distance ( pc) and orbit ( yr period) for this system composed of a low-mass star and brown dwarf. We find a moderately eccentric orbit ( ), incompatible with previous work based on less data, and dynamical masses of 99 ± 6 and 66 ± 4 for the two components. The primary mass is marginally inconsistent (2.1σ) with the empirical mass–magnitude–metallicity relation and models of main-sequence stars. The relatively high mass of the cold ( K) brown dwarf companion indicates an age older than a few gigayears, in accord with age estimates for the primary star, and is consistent with our recent estimate of ≈70 for the stellar/substellar boundary among the field population. Our improved parallax and proper motion, as well as an orbit-corrected system velocity, improve the accuracy of the system’s close encounter with the solar system by an order of magnitude. WISE J0720−0846AB passed within 68.7 ± 2.0 kau of the Sun 80.5 ± 0.7 kyr ago, passing through the outer Oort cloud where comets can have stable orbits.

BLUE STRAGGLER MASSES FROM PULSATION PROPERTIES. II. TOPOLOGY OF THE INSTABILITY STRIP

We present a new set of nonlinear, convective radial pulsation models for main sequence stars computed assuming three metallicities: Z=0.0001, 0.001 and 0.008. These chemical compositions bracket the metallicity of stellar systems hosting SX Phoenicis stars (SXPs or pulsating Blue Stragglers), namely Galactic globular clusters and nearby dwarf spheroidals. Stellar masses and luminosities of the pulsation models are based on alpha--enhanced evolutionary tracks from the BASTI website. We are able to define the topology of the instability strip (IS), and in turn the pulsation relations for the first four pulsation modes. We found that third overtones approach a stable nonlinear limit cycle. Predicted and empirical IS agree quite well in the case of 49 SXPs belonging to omega Cen. We used theoretical Period-Luminosity relations in B,V bands to identify their pulsation mode. We assumed Z=0.001 and Z=0.008 as mean metallicities of SXPs in omega Cen. We found respectively 13-15 fundamental, 22-6 first and 9-4 second overtone modes. Five are unstable in the third overtone mode only for Z=0.001. Using the above mode identification and applying the proper mass-dependent Period-Luminosity relations we found masses ranging from ~1.0 to 1.2 Mo (<M>=1.12, sigma=0.04 Mo) and from ~1.2 to 1.5 Mo (<M>=1.33, sigma=0.03 Mo) for Z=0.001 and 0.008 respectively. Our investigation supports the use of evolutionary tracks to estimate of SXP masses. We will extend our analysis to higher Helium content that may have an impact in our understanding of the BSS formation scenario.

Stellar models of multiple populations in globular clusters – I. The main sequence of NGC 6752

We present stellar atmosphere and evolution models of main sequence stars in two stellar populations of the Galactic globular cluster NGC 6752. These populations represent the two extremes of light-element abundance variations in the cluster. NGC 6752 is a benchmark cluster in the study of multiple stellar populations because of the rich array of spectroscopic abundances and panchromatic Hubble Space Telescope photometry. The spectroscopic abundances are used to compute stellar atmosphere and evolution models. The synthetic spectra for the two populations show significant differences in the ultraviolet and, for the coolest temperatures, in the near-infrared. The stellar evolution models exhibit insignificant differences in the H-R diagram except on the lower main sequence. The appearance of multiple sequences in the colour-magnitude diagrams (CMDs) of NGC 6752 is almost exclusively due to spectral effects caused by the abundance variations. The models reproduce the observed splitting and/or broadening of sequences in a range of CMDs. The ultraviolet CMDs are sensitive to variations in carbon, nitrogen, and oxygen but the models are not reliable enough to directly estimate abundance variations from photometry. On the other hand, the widening of the lower main sequence in the near-infrared CMD, driven by oxygen-variation via the water molecule, is well-described by the models and can be used to estimate the range of oxygen present in a cluster from photometry. We confirm that it is possible to use multiband photometry to estimate helium variations among the different populations, with the caveat that the estimated amount of helium-enhancement is model-dependent.

Correcting EChO data for stellar activity by direct scaling of activity signals

Stellar activity patterns are responsible for jitter effects that are observed at different timescales and amplitudes in the measurements currently obtained from photometric, spectroscopic and astrometric time series observations. These effects are usually considered just noise, and the lack of a characterization and correction strategy represents one of the main limitations for exoplanet sciences. Precise simulations of the stellar photosphere based on the most recent available models for main sequence stars can provide synthetic time series data. These may help to investigate the relation between activity jitters and stellar parameters when considering different active region patterns. Moreover, jitters can be analysed at different wavelength scales in order to design strategies to remove or minimize them from the passbands defined for specific instruments or space missions. In this work we present a model for a spotted rotating photosphere built from the contribution of a fine grid of surface elements, including all significant effects affecting the flux intensities and the wavelength of spectral features. The resulting time series data generated with this simulator are used in order to design strategies to minimize the effects of activity jitter from EChO infrared observations. An approach is presented to remove activity signals from the full infrared light curve by using the simultaneously obtained data in the visible range. Then, a similar strategy is used to correct data focused ontransit observations, and a methodology to remove the effects of non-occulted spots from transit depth measurements is presented. These procedures provide a correction of the transit data to a few times 10−5 for significantly active stars, which is within EChO noise standards.

The intermediate-age pre-cataclysmic variables SDSS J172406+562003 and RE J2013+4002

We have analyzed the physical status of the pre-cataclysmic variables SDSSJ172406+562003 and RE J2013+4002, which have evolved after their common-envelope stage a time t = 106−107 years. Spectroscopy and photometry of these systems were performed with the 6-m and 1-m telescopes of the Special Astrophysical Observatory. We demonstrate that emission lines in the spectra were formed solely by the reflection of radiation emitted by the white dwarfs on the surfaces of their cool companions, under conditions close to local thermodynamic equilibrium. These effects are also responsible for most of the objects’ photometric variability amplitude. However, comparing the light curves of SDSS 172406 from different epochs, we find aperiodic brightness variations, probably due to spottedness of the surface of the secondary. Jointly analyzing the spectra, radial-velocity curves, and light curves of the pre-cataclysmic variables and modeling the reflection effects, we have derived their fundamental parameters. We demonstrate that the secondaries in these systems are consistent with evolutionary models for main-sequence stars and do not have the luminosity excesses characteristic of cool stars in young pre-cataclysmic variables.

Dark stars at the Galactic Centre - the main sequence

In regions of very high dark matter density such as the Galactic centre, the capture and annihilation of WIMP dark matter by stars has the potential to significantly alter their evolution. We describe the dark stellar evolution code DarkStars, and present a series of grids of WIMP-influenced stellar models for main sequence stars. We describe changes in which occur as a function of the rate of energy injection by WIMPs, for stars of 0.3-2.0 solar masses and metallicities Z = 0.0003-0.02. We show what rates of energy injection can be obtained using realistic orbital parameters for stars at the Galactic centre, including detailed consideration of the velocity and density profiles of dark matter. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits. If there is a spike of dark matter induced by the supermassive black hole at the Galactic centre, single solar-mass stars following orbits with periods as long as 50 years and eccentricities as low as 0.9 could be significantly affected. Binary systems with similar periods about the Galactic centre could be affected on even less eccentric orbits. The most striking evidence of this scenario would be the existence of a binary consisting of a low-mass protostar and a higher-mass evolved star. The observation of low-mass stars and/or binaries on such orbits would either provide a detection of WIMP dark matter, or place stringent limits on the combination of the WIMP mass, spin-dependent nuclear-scattering cross-section, halo density and velocity distribution near the Galactic centre. In some cases, the limits on the WIMP mass and spin-dependent nuclear-scattering cross-section would be of comparable sensitivity to current direct-detection experiments.

Magnetic Star Models with Toroidal Component of Field

A variety of models of main sequence stars of a few solar masses containing large scale magnetic fields have been published in the last few years. Most of them have contained purely poloidal magnetic fields (with magnetic and rotation axes aligned). Those containing a toroidal field component were either polytropic models, or, if more realistic, the toroidal field was not well behaved at infinity.

Effect of Radiation Pressure on Stellar Models.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effect of Radiation Pressure on Stellar Models. Auman, J. R. ; Bahng, J. Abstract A study has been made of the effect of including radiation pressure in the calculation of interior models for main sequence stars in the mass range from 2 to 5 solar masses. The inclusion of radiation pressure reduces the total luminosity slightly, but the radius remains practically unchanged. The largest effect is found in the size of the convective core; it is increased substantially when the radiation pressure is taken into account. Although the resulting changes on the structure of main sequence stars are quite small, the reduced luminosity and increased convective core will appreciably lengthen the time scale of subsequent evolution of these stars. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109592 Bibcode: 1965AJ.....70R.318A full text sources ADS |

Models for Main Sequence Stars

Computations for main sequence models have been made using the method of Haselgrove and Hoyle (1) for uniform compositions ranging from 76 per cent to 99 per cent hydrogen (by mass). Early stages of the evolution of the models with initial composition of 85 per cent hydrogen are also discussed, and finally a model for the Sun with initial composition 76.2 per cent hydrogen is given.