M3: Inconstant Star Cluster

NGC 5272 :: new link

APOD: 2007 April 15 – M3: Inconstant Star Cluster:

Star clusters appear constant because photographs of them are frozen in time. In reality, though, cluster stars swarm the center and frequently fluctuate in brightness. Although the time it takes for stars to cross a cluster is about 100,000 years, the time it takes for a star to fluctuate noticeably can be less than one night. In fact, the above time lapse movie of bright globular cluster M3 was taken over a single night…

The RR Lyrae period–K-luminosity relation for globular clusters: an observational approach

New Paper :: Variable Stars

Monthly Notices of the Royal Astronomical Society
“Online Early”
doi:10.1111/j.1365-2966.2006.10962.x

A. Sollima,  C. Cacciari and E. Valenti
“The RR Lyrae period–K-luminosity relation for globular clusters: an observational approach”

Abstract:
The period–metallicity–K-band luminosity (PL_K) relation for RR Lyrae stars in 15 Galactic globular clusters and in the Large Magellanic Cloud (LMC) globular cluster Reticulum has been derived. It is based on accurate near-infrared (K) photometry combined with Two-Micron All-Sky Survey (2MASS) and other literature data. The PL_K relation has been calibrated and compared with the previous empirical and theoretical determinations in literature. The zero point of the absolute calibration has been obtained from the K magnitude of RR Lyr whose distance modulus has been measured via trigonometric parallax with Hubble Space Telescope (HST). Using this relation, we obtain a distance modulus to the LMC of (m − M)₀ = 18.54 ± 0.15 mag, in good agreement with recent determinations based on the analysis of Cepheid variable stars.

Link to the MNRAS website
(access restriction applies for the full article)

RR Lyrae Period – K Luminosity relation for GCs…

Preprint (astro-ph/060837)

The RR Lyrae Period – K Luminosity relation for Globular Clusters: an observational approach

Authors: A. Sollima, C. Cacciari, E. Valenti
Comments: 6 pages, 4 figures, accepted for publication by MNRAS

The Period – metallicity – K band luminosity (PLK) relation for RR Lyrae stars in 15 Galactic globular clusters and in the LMC globular cluster Reticulum has been derived. It is based on accurate near infrared (K) photometry combined with 2MASS and other literature data. The PLK relation has been calibrated and compared with the previous empirical and theoretical determinations in literature. The zero point of the absolute calibration has been obtained from the K magnitude of RR Lyr whose distance modulus has been measured via trigonometric parallax with HST. Using this relation we obtain a distance modulus to the LMC of (m-M)_0 = 18.54 \pm 0.15 mag, in good agreement with recent determinations based on the analysis of Cepheid variable stars.

http://arxiv.org/abs/astro-ph/0608397

A pulsational distance to Omega Centauri…

Preprint (astro-ph/0608052)

Authors: M. Del Principe, A. M. Piersimoni, J. Storm, F. Caputo, G. Bono, P. B. Stetson, M. Castellani, R. Buonanno, A. Calamida, C. E. Corsi, M. Dall’Ora, I. Ferraro, L. M. Freyhammer, G. Iannicola, M. Monelli, M. Nonino, L. Pulone, V. Ripepi
Comments: 24 pages, 7 figures, accepted for publication on The Astrophysical Journal

“A pulsational distance to Omega Centauri based on Near-Infrared Period-Luminosity relations of RR Lyrae stars”

We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the globular cluster Omega Cen (NGC 5139) which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is presently the largest such sample in any stellar system. We also present updated predictions for J,K-band Period-Luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex) with respect to iron we find a true distance modulus of 13.70 (with a random error of 0.06 and a systematic error of 0.06), corresponding to a distance d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC-17, but differ significantly from the recent distance estimates based on cluster dynamics and on high amplitude Delta Scuti stars.

http://babbage.sissa.it/abs/astro-ph/0608052

Image-Subtraction Photometry of Variable Stars in NGC 6388 and NGC 6441

New paper (“The Astronomical Journal”, Volume 132):

Corwin, T. Michael; Sumerel, Andrew N.; Pritzl, Barton J.; Smith, Horace A.; Catelan, M.; Sweigart, Allen V.; Stetson, Peter B.

Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441

We have applied Alard’s image-subtraction method (ISIS ver. 2.1) to the observations of the globular clusters NGC 6388 and NGC 6441 previously analyzed using standard photometric techniques (DAOPHOT and ALLFRAME). In this reanalysis of observations obtained at the Cerro Tololo Inter-American Observatory, besides recovering the variables previously detected on the basis of our ground-based images, we have also been able to recover most of the RR Lyrae variables previously detected only in the analysis of Hubble Space Telescope WFPC2 observations of the inner region of NGC 6441. In addition, we report five possible new variables not found in the analysis of the Hubble Space Telescope observations of NGC 6441. This dramatically illustrates the capabilities of image-subtraction techniques applied to ground-based data to recover variables in extremely crowded fields. We have also detected 12 new variables and 6 possible variables in NGC 6388 not found in our previous ground-based studies. The revised mean period for RRab stars in NGC 6388 is 0.676 days, while the mean period of RRab stars in NGC 6441 is unchanged at 0.759 days. These values are among the largest known for any Galactic globular cluster. Additional probable type II Cepheids were identified in NGC 6388, confirming its status as a metal-rich globular cluster rich in Cepheids.

ADS link to the paper

(mcastel)

The period-luminosity relation for type II Cepheids in GCs

New paper:

Matsunaga, Noriyuki et al.

The period-luminosity relation for type II Cepheids in globular clusters

We report the result of our near-infrared observations (JHK_s) for type II Cepheids (including possible RV Tau stars) in galactic globular clusters. We detected variations of 46 variables in 26 clusters (10 new discoveries in seven clusters) and present their light curves. Their periods range from 1.2 d to over 80 d. They show a well-defined period-luminosity relation at each wavelength. Two type II Cepheids in NGC 6441 also obey the relation if we assume the horizontal branch stars in NGC 6441 are as bright as those in metal-poor globular clusters in spite of the high metallicity of the cluster. This result supports the high luminosity which has been suggested for the RR Lyr variables in this cluster (…)

Publication: Monthly Notices of the Royal Astronomical Society, Online Early

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