704 research outputs found
Unusual ferromagnetism in Ising and Potts model on semi-directed Barab\'asi-Albert networks
We check the existence of a spontaneous magnetisation of Ising and Potts
spins on semi-directed Barabasi-Albert networks by Monte Carlo simulations. We
verified that the magnetisation for different temperatures decays after a
characteristic time , which we extrapolate to diverge at positive
temperatures by a Vogel-Fulcher law, with increasing
logarithmically with network size .Comment: 14 pages, 8 figure
The variable star population in the globular cluster NGC 6934
We report an analysis of new and CCD time-series photometry of the
globular cluster NGC 6934. Through the Fourier decomposition of the RR Lyrae
light curves, the mean values of [Fe/H] and the distance of the cluster were
estimated, we found; [Fe/H]=-1.480.14 and =16.030.42 kpc,
and [Fe/H]=-1.430.11 and =15.910.39 kpc, from the
calibrations of RRab and RRc stars respectively. Independent distance
estimations from SX Phe and SR stars are also discussed. Individual absolute
magnitudes, radii and masses are also reported for RR Lyrae stars. We found 12
new variables: 4 RRab, 3 SX Phe, 2 W Virginis (CW)and 3 semi-regular (SR). The
inter-mode or "either-or" region in the instability strip is shared by the RRab
and RRc stars. This characteristic, observed only in some OoI clusters and
never seen in an OoII, is discussed in terms of mass distribution in the ZAHB.Comment: 25 pages, 16 figures, 6 table
New variables in M5 (NGC 5904) and some identification corrections
We report twelve variables not previously detected in the globular cluster M5
(NGC 5904); one SX Phe and eleven semi-regular variables (SR). Their
identifications, equatorial coordinates, ephemerides, and light curves are
given. Furthermore, we have explored the light curves of a group of stars whose
variability has not been confirmed and that are marked as probable non-
variables in the CVSGC. Finally, we offer detailed identifications for some of
the known variables in crowded regions that were misidentified in previous
studies. We shall also address the cases of the cataclysmic variable or U Gem
type V101 and of the variable blue straggler V159.Comment: 10 pages, 4 figures, 5 table
Identification of metal-poor stars using the artificial neural network
Identification of metal-poor stars among field stars is extremely useful for
studying the structure and evolution of the Galaxy and of external galaxies. We
search for metal-poor stars using the artificial neural network (ANN) and
extend its usage to determine absolute magnitudes. We have constructed a
library of 167 medium-resolution stellar spectra (R ~ 1200) covering the
stellar temperature range of 4200 to 8000 K, log g range of 0.5 to 5.0, and
[Fe/H] range of -3.0 to +0.3 dex. This empirical spectral library was used to
train ANNs, yielding an accuracy of 0.3 dex in [Fe/H], 200 K in temperature,
and 0.3 dex in log g. We found that the independent calibrations of near-solar
metallicity stars and metal-poor stars decreases the errors in T_eff and log g
by nearly a factor of two. We calculated T_eff, log g, and [Fe/H] on a
consistent scale for a large number of field stars and candidate metal-poor
stars. We extended the application of this method to the calibration of
absolute magnitudes using nearby stars with well-estimated parallaxes. A better
calibration accuracy for M_V could be obtained by training separate ANNs for
cool, warm, and metal-poor stars. The current accuracy of M_V calibration is
(+-)0.3 mag. A list of newly identified metal-poor stars is presented. The M_V
calibration procedure developed here is reddening-independent and hence may
serve as a powerful tool in studying galactic structure.Comment: Accepted for publication in Astronomy and Astrophysics, 15 pages, 7
figure
Spectroscopic analysis of four post-AGB candidates
We have done a detailed abundance analysis of four unexplored candidate post-
Asymptotic Giant Branch(AGB) stars IRAS 13110 - 6629, IRAS 17579 - 3121, IRAS
18321 - 1401 and IRAS 18489 - 0629 using high resolution spectra. We have
constructed Spectral Energy Distributions (SED) for these objects using the
existing photometric data combined with infrared (IR) fluxes. For all sample
stars, the SEDs exhibit double peaked energy distribution with well separated
IR peaks showing the presence of dusty circumstellar material. The CNO
abundances indicate the production of N via CN cycling, but observed [C/Fe]
indicates the mixing of carbon produced by He burning by third dredge up
although C/O ratio remains less that 1. A moderate DG effect is clearly seen
for IRAS 18489 - 0629 and IRAS 17579 - 3121 while a large scatter observed in
depletion plots for IRAS 18321 - 1401 and IRAS 13110 - 6629 indicate the
presence of other processes affecting the observed abundance pattern.Comment: 15, pages, 5 tables, 9 Figures. Accepted in Revista Mexicana de
Astronom\'ia y Astrof\'isica, Octuber issue 201
Unveiling Vela - Time Variability of Na I D lines in the Direction of the Vela Supernova Remnant
High-resolution spectral profiles of Na I D lines from the interstellar
medium towards 64 stars in the direction of the Vela supernova remnant are
presented. This survey conducted mostly between 2011-12 complements an earlier
survey of the same stars by Cha & Sembach done in the 1993-96 period. The
interval of 15 to 18 years provides a base line to search for changes in the
interstellar profiles. Dramatic disappearance of strong absorption components
at low radial velocity is seen towards three stars - HD 63578, HD 68217, HD
76161 - over 15-18 years; HD 68217 and HD 76161 are associated with the Vela
SNR but HD 63578 is likely associated with the wind bubble of g2 Velorum. The
vanishing of these cold neutral clouds in the short time of 15 to 18 years
needs some explanation. Other changes are seen in high-velocity Na D
components.Comment: 26 pages, 20 figures, 7 tables, accepted in MNRA
Physical parameters of RR Lyrae stars in NGC 6171
We present an analysis of VI CCD time-series photometry of the globular
cluster NGC 6171. The main goal is to determine individual physical parameters
for single-mode RR Lyrae stars present in the field of view of our images, by
means of light curve Fourier decomposition and well-established calibrations
and zero-points. This leads to the estimation of the mean values of the
metallicity and distance for the parental cluster. From the RRab stars we find
[Fe/H]ZW = -1.33 +- 0.12 and a distance d = 5.3 +- 0.3 kpc and from the RRc
stars we find [Fe/H]ZW = -1.02 +- 0.19 and a distance d = 5.3 +- 0.2 kpc.
Independent methods, such as the P-L relations for RRab and SX Phe stars enable
the estimation of a weighted average distance to the cluster of 5.4 +- 0.1 kpc.
We confirm the amplitude modulations of the Blazhko type of five RRab and the
non-membership of V21 to the cluster. The colour-magnitude diagram is
consistent with an age of 11 Gyrs. The distribution of RRab and RRc stars seems
well segregated around the first overtone red edge of the instability strip.
This positions NGC 6171 among OoI type clusters where the pulsating modes are
neatly separated in the HB. We report two new irregular variables of the Lb
type.Comment: 13 pages, 8 figures, accepted on October 4, 201
The Stellar parametrization using Artificial Neural Network
An update on recent methods for automated stellar parametrization is given.
We present preliminary results of the ongoing program for rapid parametrization
of field stars using medium resolution spectra obtained using Vainu Bappu
Telescope at VBO, Kavalur, India. We have used Artificial Neural Network for
estimating temperature, gravity, metallicity and absolute magnitude of the
field stars. The network for each parameter is trained independently using a
large number of calibrating stars. The trained network is used for estimating
atmospheric parameters of unexplored field stars.Comment: 6 pages, 2 figures, To appear in Astronomical Society of India
Conference Series (ASICS). eds. P. Prugniel and H.P Sing
Period changes in the RR Lyrae stars of NGC 6171 (M107)
Based on photometric data obtained between 1935 and 2017, diagrams were
built for 22 RR Lyrae stars in the globular cluster NGC 6171, leading to the
discovery of secular period changes in 4 variables for which we have calculated
their period change rates . In contrast we find that of the
sample stars have stable periods over the last 82 years. For the stable period
stars, the whole data base has been employed to refine their periods. Among the
period changing stars, three (V10, V12 and V16) have decreasing periods larger
than expected from stellar evolution. Despite these individual cases of
significant period change rate, the golbal average of the measured period
changes in the cluster is basically zero, in consonance with theoretical
predictions for clusters with redder horizontal branches. The hitherto
unpublished observations, now brought into public domain, are employed to
calculate a set of times of maximum light which are used in the present
analysis.Comment: 10 pages, 3 figures, 6 tables, full Tables 2 and 4 will be electroni
The curious case of PDS 11: a nearby, >10 Myr old, classical T Tauri binary system
We present results of our study of PDS 11 binary system, which belongs to a
rare class of isolated, high galactic latitude T Tauri stars. Our spectroscopic
analysis reveals that PDS 11 is a M2-M2 binary system with both components
showing similar H-alpha emission strength. Both the components appear to be
accreting, and are classical T Tauri stars. The lithium doublet Li I 6708
Angstrom, a signature of youth, is present in the spectrum of PDS 11A, but not
in PDS 11B. From the application of lithium depletion boundary age-dating
method and a comparison with the Li I 6708 equivalent width distribution of
moving groups, we estimated an age of 10-15 Myr for PDS 11A. Comparison with
pre-main sequence evolutionary models indicates that PDS 11A is a 0.4 solar
mass T Tauri star at a distance of 114-131 pc. PDS 11 system does not appear to
be associated with any known star forming regions or moving groups. PDS 11 is a
new addition, after TWA 30 and LDS 5606, to the interesting class of old,
dusty, wide binary classical T Tauri systems in which both components are
actively accreting.Comment: Accepted for publication in The Astronomical Journal (13 pages, 10
figures
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