188 research outputs found
RR Lyrae Variables in the Globular Cluster M5
We present V-band CCD photometry of 65 RR Lyr variables from the globular
cluster M5. We have estimated the basic physical parameters for 16 RRc stars
and 26 RRab stars using a Fourier decomposition of the light curves of the
variables. The mean values of mass, luminosity, effective temperature and
relative helium abundance for the RRc stars are measured to be , log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively.
For the RRab variables the derived mean values of absolute magnitude,
metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and
T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period
is a function of metal abundance rather than Oosterhoff type. We find
significant problems with the calibration of both the zero point and the scale
of the luminosities measured with the Fourier technique. The apparent distance
modulus derived from RRc stars is equal to and it is in good
agreement with recent determinations. On the other hand distance modulus
obtained from the sample of RRab stars (calibrated by the Baade-Wesselink
observations of field RR Lyr variables) is significantly smaller and equal to
Comment: accepted for publication in Astronomy and Astrophysics, 15 pages with
6 figure
Clusters AgeS Experiment. RR Lyr Variables in the Globular Cluster NGC 6362
We present V and B-band CCD photometry of 35 RR Lyr stars in the southern
hemisphere globular cluster NGC 6362. Fourier decomposition of the light curves
was used to estimate the basic properties of these variables. From the analysis
of the RRc stars we obtain a mean mass of M=0.531+/-0.014 , luminosity
log L=1.656+/-0.006, effective temperature Teff=7429+/-20, and helium abundance
Y=0.292+/-0.002. The mean values of the absolute magnitude, metallicity (on
Jurcsik's scale) and effective temperature for 14 RRab stars with "regular"
light curves are: M_V=0.86+/-0.01, [Fe/H]=-0.93+/-0.04 and Teff=6555+/-25,
respectively. From the B-V colors, periods and metallicities of the RRab stars
we estimate the color excess for NGC 6362 to be equal to E(B-V)=0.08+/-0.01.
Using this value we derive the colors of the blue and red edges of the
instability strip in NGC 6362 to be and
. From the relations between the Fourier coefficients of
RRab and RRc stars and their absolute magnitudes we estimate the apparent
distance modulus to NGC 6362 to be (m-M)_V=14.46+/-0.10. From the mean value of
log L of the RRc stars we obtain 14.59+/-0.03. The V-band light curves of three
of the RRc stars exhibit changes in amplitude of over 0.1 mag on the time scale
of around one week. Near the radial 1st overtone frequency we find one or two
peaks, which strongly suggests that these variables belong to the newly
identified group of non-radial pulsating RR Lyr stars.Comment: 17 pages, 9 figures, submitted to MNRA
Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars
We find that low-degree low-order g-modes become unstable in metal-poor
low-mass stars due to the -mechanism of the pp-chain. Since the
outer convection zone of these stars is limited only to the very outer layers,
the uncertainty in the treatment of convection does not affect the result
significantly. The decrease in metallicity leads to decrease in opacity and
hence increase in luminosity of a star. This makes the star compact and results
in decrease in the density contrast, which is favorable to the
-mechanism instability. We find also instability for high order
g-modes of metal-poor low-mass stars by the convective blocking mechanism.
Since the effective temperature and the luminosity of metal-poor stars are
significantly higher than those of Pop I stars, the stars showing
Dor-type pulsation are substantially less massive than in the case of Pop I
stars. We demonstrate that those modes are unstable for about
stars in the metal-poor case.Comment: 4 pages, 4 figures, To be published in Astrophysics and Space Science
Proceedings series (ASSP). Proceedings of the "20th Stellar Pulsation
Conference Series: Impact of new instrumentation and new insights in stellar
pulsations", 5-9 September 2011, Granada, Spai
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