SPITZERSPECTROSCOPY OF MASS-LOSS AND DUST PRODUCTION BY EVOLVED STARS IN GLOBULAR CLUSTERS

Abstract

We have observed a sample of 35 long-period variables (LPVs) and four Cepheid variables in the vicinity of 23 Galactic globular clusters using the Infrared Spectrograph on the Spitzer Space Telescope. The LPVs in the sample cover a range of metallicities from near solar to about 1/40th solar. The dust mass-loss rate (MLR) from the stars increases with pulsation period and bolometric luminosity. Higher MLRs are associated with greater contributions from silicate grains. The dust MLR also depends on metallicity. The dependence is most clear when segregating the sample by dust composition, less clear when segregating by bolometric magnitude, and absent when segregating by period. The spectra are rich in solid-state and molecular features. Emission from alumina dust is apparent across the range of metallicities. Spectra with a 13 μm dust emission feature, as well as an associated feature at 20 μm, also appear at most metallicities. Molecular features in the spectra include H 2 O bands at 6.4-6.8 μm, seen in both emission and absorption, SO 2 absorption at 7.3-7.5 μm, and narrow emission bands from CO 2 from 13.5 to 16.8 μm. The star Lyngå 7 V1 has an infrared spectrum revealing it to be a carbon star, adding to the small number of carbon stars associated with Galactic globular clusters.

The VLT/VISIR mid-IR view of 47 Tucanae

“…Indeed, the 13 μm feature (very similar to that of SAO 37673) is commonly found in nonMira semi-regular variables according to Speck et al (2000). On the other hand, following Sloan et al (2010), the same 13 μm feature is detected in Miras. This agrees with the results presented in Lebzelter & Wood (2005), showing that the V8 velocity curve resembles those of typical Miras found in the solar neighbourhood.…”

A Carbon-rich Mira variable in a globular cluster: A stellar merger

“…In such cases the amount of mass exchanged is likely to have been relatively limited and sufficient only to pollute the stellar atmosphere. Sloan et al (2010) suggest that V1 could be a CH type object. However, with a predicted initial mass of ∼ 1.5 solar masses this is not possible.…”

“…The variable has a period of 551 days, a large infrared amplitude (∆K = 1.22 mag) and very red colours (mean (J −K) = 4.1 mag) indicative of mass-loss and a circumstellar dust shell. Sloan et al (2010) obtained SPITZER mid-infrared spectra of the variable showing it to be a carbon star with strong circumstellar dust emission from SiC and MgS particles as well as absorption from gaseous acetylene. They estimate a mass loss of 2.5×10 −7 solar masses per year.…”

The VVV Templates Project Towards an automated classification of VVV light-curves

“…2 http://irsa.ipac.caltech.edu/ 3 http://www.saao.ac.za/~isg/irsf.html There have been several extensive IRSF/SIRIUS surveys of variable stars in various regions: the Magellanic Clouds (Ita et al 2004a(Ita et al ,b, 2009, the Galactic Center (Matsunaga et al 2009(Matsunaga et al , 2011, globular clusters (Matsunaga et al 2006;Sloan et al 2010), and Local Group dwarf galaxies (Whitelock et al 2013, and references therein). Considering the numbers of monitoring epochs, the surveys discussed below provide us with particularly useful datasets to construct template light-curves.…”