Category Archives: Article

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 (PLK) 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 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 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)

HST astrometry and VLT radial velocities in NGC 6397

NGC 6397 :: New published paper

A. P. Milone et al.

Absolute motions of globular clusters

II. HST astrometry and VLT radial velocities in NGC 6397

 
Abstract
In this paper we present a new, accurate determination of the three components of the absolute space velocity of the Galactic globular cluster NGC 6397 ( $l\simeq338^\circ$, $b\simeq-12^\circ$). We used three HST/WFPC2 fields with multi-epoch observations to obtain astrometric measurements of objects in three different fields in this cluster. The identification of 33 background galaxies with sharp nuclei allowed us to determine an absolute reference point and measure the absolute proper motion of the cluster. The third component was obtained from radial velocities measured from spectra from the multi-fiber spectrograph FLAMES at UT2-VLT. We find ( $\mu_\alpha \cos{\delta}$, $\mu_\delta$) $_{\rm J2000.0}$ = ( $+3.39 \pm 0.15$, $-17.55 \pm 0.15$) mas yr-1 and $V_{\rm rad} = +18.36 \pm 0.09$ ($\pm$0.10) km s-1. Assuming a Galactic potential, we calculate the cluster orbit for various assumed distances and briefly discuss the implications.

A&A 456 (2006) 517-522 (Section ‘Galactic structure, stellar clusters, and populations’)

Astronomy & Astrophysics website

Hot horizontal-branch stars in the metal-rich bulge globular cluster NGC6388…

NGC 6388 :: New published paper

S. Moehler and A. V. Sweigart

Observations of the hot horizontal-branch stars in the metal-rich bulge globular cluster NGC6388 – Indications of helium enrichment and a lesson in crowded field spectroscopy.

A&A 455 (2006) 943-952
(Section ‘Galactic structure, stellar clusters, and populations’)

http://publish.edpsciences.org/abstract/aa/v455/p943

Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tuc…

47 Tucanae (NGC 104) :: New published paper

The Astrophysical Journal Letters
Volume 647, Number 1
(August 10 2006)

F. R. Ferraro, E. Sabbi, R. Gratton, G. Piotto, B. Lanzoni, E. Carretta, R. T. Rood, Alison Sills, F. Fusi Pecci, S. Moehler, G. Beccari, S. Lucatello, and N. Compagni
“Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process”

Page L53 [ http://www.journals.uchicago.edu/cgi-bin/resolve?ApJL20841 ]

Abstract:
We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.

Table of Contents of ApL Volume 647, Number 1:
http://www.journals.uchicago.edu/cgi-bin/contents?ApJL+v647n1

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 (JHKs) 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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A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in 47 Tuc

New article on ApJ:

Heinke, Craig O.; Rybicki, George B.; Narayan, Ramesh; Grindlay, Jonathan E.
“A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae”

Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This has motivated us to calculate new hydrogen atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and self-irradiation by photons from the compact object. We have constructed a self-consistent grid of neutron star models covering a wide range of surface gravities, as well as effective temperatures, which we make available to the scientific community. We present multiepoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fitted by our hydrogen atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 Msolar is assumed, our spectral fits require that its radius be in the range Rns=14.5+1.8-1.6 km (90% confidence), which is larger than that expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of Mns=2.20+0.03-0.16 Msolar is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.

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Merging of Globular Clusters in Inner Galactic Regions. I.

    Miocchi, P.; Capuzzo Dolcetta, R.; Di Matteo, P.; Vicari, A.

Merging of Globular Clusters in Inner Galactic Regions. I. Do They Survive the Tidal Interaction?

The Astrophysical Journal, Volume 644, Issue 2, pp. 940-953.

The main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N-body simulations, whose first results are presented in this article. Our simulations dealt with primordial, very massive (of the order of 107 Msolar) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a timescale much shorter than their internal relaxation time….

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