arXiv subjects

V. Silva Aguirre

Publications and source records attributed to V. Silva Aguirre.

77 records · Page 5Linked to original sources

arXiv2010

A precise asteroseismic age and radius for the evolved Sun-like star KIC 11026764

The primary science goal of the Kepler Mission is to provide a census of exoplanets in the solar neighborhood, including the identification and characterization of habitable Earth-like planets. The asteroseismic capabilities of the mission are being used to determine precise radii and ages for the target stars from their solar-like oscillations. Chaplin et al. (2010) published observations of three bright G-type stars, which were monitored during the first 33.5 days of science operations. One of these stars, the subgiant KIC 11026764, exhibits a characteristic pattern of oscillation frequencies suggesting that it has evolved significantly. We have derived asteroseismic estimates of the properties of KIC 11026764 from Kepler photometry combined with ground-based spectroscopic data. We present the results of detailed modeling for this star, employing a variety of independent codes and analyses that attempt to match the asteroseismic and spectroscopic constraints simultaneously. We determine both the radius and the age of KIC 11026764 with a precision near 1%, and an accuracy near 2% for the radius and 15% for the age. Continued observations of this star promise to reveal additional oscillation frequencies that will further improve the determination of its fundamental properties.

astro-ph.SR↗

arXiv2010

Convective core size determination using asteroseismic data

As it is long known, the presence of a convective region creates a discontinuity in the chemical profile of a star, which in turn translates into a sharp variation of the adiabatic sound speed. This variation produces an oscillatory behavior of the pulsation frequencies, to which low degree p-modes are sensitive. We investigate the possibility of detecting the signature related to the presence of a convective core in the frequency spectrum of low-mass stars by means of suitable frequency combinations (such as separations and ratios)

astro-ph.SR↗

arXiv2010

Semiconvective Mixing in Low-Mass Stars

Mixing processes such as convection, overshooting and rotational mixing have long been known to affect the evolutionary properties of low-mass stars. While modeling a 1.2 Msun star, we encountered a semiconvective region outside the fully convective core, reminiscent of the well-known situation for massive stars. In this study we focus on low-mass stars presenting convective cores and, by applying different prescriptions for the determination of the convective boundaries and using different mixing descriptions for the dynamical processes in the core, we look for the effects of semiconvective mixing in the interior structure of the stars and its observable quantities. With this purpose, we have constructed different sets of evolutionary models using a stellar evolution code (GARSTEC), and analyzed the models looking for imprints of these processes.

astro-ph.SR↗

arXiv2009

Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars

One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the red giant branch (RGB) and is followed by a series of secondary flashes before the star settles into the zero-age horizontal branch (ZAHB). Yet, no stars have been positively identified in this key evolutionary phase, mainly for two reasons: first, this pre-ZAHB phase is very short compared to other major evolutionary phases in the life of a star; and second, these pre-ZAHB stars are expected to overlap the loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in the color-magnitude diagram (CMD). We investigate the possibility of detecting these stars through stellar pulsations, since some of them are expected to rapidly cross the Cepheid/RR Lyrae instability strip in their route from the RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a consequence of their very high evolutionary speed, some of these stars may present anomalously large period change rates. We constructed an extensive grid of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR Lyrae stars with high period change rates are found. Our results suggest that some -- but certainly not all -- of the RR Lyrae stars in M3 with large period change rates are in fact pre-ZAHB pulsators.

astro-ph.SR↗

arXiv2008

Stellar Evolution and Variability in the Pre-ZAHB Phase

One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the RGB and is followed by a series of secondary flashes before the star settles on the zero-age HB (ZAHB). Yet, no stars have ever been positively identified in this key phase in the life of a low-mass star (hereafter the "pre-ZAHB" phase). We investigate the possibility that some pre-ZAHB stars may cross the instability strip (IS), thus becoming variable. In particular, it has been suggested that some RR Lyrae stars (RRL) with high period change rates (P-dot) may in fact be pre-ZAHB stars. We present the first theoretical effort devoted to interpreting some of the high-P-dot stars as pre-ZAHB pulsators. We constructed a grid of evolutionary tracks using the Garching Stellar Evolution Code for a chemical composition appropriate to the case of the globular cluster M3, where a number of stars with high P-dot values are found. We follow each star's pre-ZAHB evolution in detail, compute periods and P-dot values for the stars lying inside the IS, and produce pre-ZAHB Monte Carlo simulations for the case of M3. Our results indicate that one should expect of order 1 pre-ZAHB star for every 60 or so bona-fide HB stars in M3. Among the pre-ZAHB stars, ~22% are expected to fall within the IS boundaries, presenting RRL-like pulsations. On average, the pre-ZAHB pulsators are expected to have longer periods than the bona-fide HB pulsators, and 76% of them are predicted to show negative P-dot values. The most likely P-dot value for the pre-ZAHB variables is ~-0.3 d/Myr, but more extreme P-dot values are also possible: 38% of the variables are predicted to have P-dot < -0.8 d/Myr. It appears likely, therefore, that some RRL stars in M3 with high (absolute) P-dot values are pre-ZAHB pulsators in disguise. (abridged)

astro-ph↗