7 research outputs found
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.Comment: Conference Proceedings HELAS Workshop on 'Synergies between solar and
stellar modelling', Rome, June 2009, Astrophys. Space Sci., in the pres
Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808: Direct spectroscopic measurements
We present an abundance analysis of 96 horizontal branch (HB) stars in NGC 2808, a globularcluster exhibiting a complex multiple stellar population pattern. These stars are distributed indifferent portions of the HB and cover a wide range of temperature
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
The galactic globular cluster M5 (NGCÂ 5904) as a particle physics laboratory
Globular clusters have been used for a long time to test stellar evolution theories, and in particular to constrain novel forms of energy loss in low-mass stars. This includes constraints on axion properties, neutrino dipole moments, milli-charged particles, Kaluza-Klein gravitons, and many other phenomena. Depending on their interaction strength, these particles can be abundantly produced in stellar interiors, escape without further interaction, and thus drain energy directly from the stellar interior. Hence, they contribute directly to the stellar energy losses, thus modifying stellar evolution. Our goal is to re-examine such constraints in the light of modern data and updated stellar evolution codes, paying particular attention to systematic and statistical errors. As a first example, we consider the case of a neutrino magnetic moment that enhances the energy loss from the plasma process.
In terms of the observed color-magnitude diagrams, the tip of the red giant branch (RGB) has been identified as a sensitive observable of the effects of the energy losses due to a neutrino magnetic moment. Here we describe the consequences of adding the cooling effect due to a neutrino magnetic moment to the Princeton-Goddard-PUC (PGPUC) stellar evolution code, exploring in particular the dependence of the position of the RGB tip on the neutrino magnetic moment.
As a first application, we studied the position of the observed RGB tip in the case of the Galactic globular cluster M5 (NGC 5904), using the latest, high-precision, ground-based data from the P. B. Stetson database (2012, priv. comm.). We compare the empirical results with the PGPUC model predictions, and discuss the implied constraints on the value of the neutrino magnetic moment