NGC 1261: A Time-Series Vi Study of Its Variable Stars

We present an analysis of VI CCD time series photometry of globular cluster NGC 6712. Our main goal is to study the variable star population as indicators of the cluster mean physical parameters. We employed the Fourier decomposition of RR Lyrae light curves to confirm that [Fe/H] UVES = −1.0 ± 0.05 is a solid estimate. We estimated the reddening to the cluster as E(B − V) = 0.35 ± 0.04 from the RRab stars colour curves. The distance to the cluster was estimated using three independent methods which yielded a weighted mean distance d = 8.1 ± 0.2 kpc. The distribution of RRab and RRc stars on the HB shows a clear segregation around the first overtone red edge of the instability strip, which seems to be a common feature in OoI-type cluster with a very red horizontal branch. We carried out a membership analysis of 60,447 stars in our FoV using the data from Gaia-DR2 and found 1529 likely members; we possess the light curves of 1100 among the member stars. This allowed us to produce a clean colour-magnitude diagram, consistent with an age of 12 Gyrs, and enabled us to discover close unresolved contaminants for several variable stars. From the proper motion analysis we found evidence of non-member stars in the FoV of the cluster being tidally affected by the gravitational pull of the bulge of the Galaxy. We found that the RRab variable V6, shows a previously undetected Blazhko effect. Finally, we report sixteen new variables of the EW-type (9) and SR-type (7).

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NGC 6712: the variable star population of a tidally disrupted globular cluster

Deras

Ferro

Lázaro

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Study of UV-bright stellar populations in the globular cluster NGC 1261 using Astrosat

Rani

Pandey

Subramaniam

et al. 2021

Monthly Notices of the Royal Astronomical Society

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We present UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on board Astrosat. We performed point-spread function (PSF) photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour–magnitude diagrams (CMDs), in combination with the Hubble Space Telescope (HST), Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in the NUV and blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV–optical CMDs, well fitted with isochrones generated (age 12.6 Gyr, [Fe/H] = −1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (Teff), luminosities, and radii of bright HB stars were estimated using the spectral energy distribution. As we detect the complete sample of UV-bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump ($T_{\rm eff}\, \sim$ 11500 K). The two EHB stars, fitted well with single spectra, have Teff = 31000 K and a mass = 0.495 M⊙, and follow the same Teff–radius relation as the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (due either to rotation or enhanced helium) or a early hot-flash scenario.

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CCD VI time-series of the extremely metal-poor globular cluster M92: revisiting its variable star population

Yepez

Ferro

Deras

2020

Monthly Notices of the Royal Astronomical Society

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We present an analysis of VI CCD time-series photometry of the Oo II type globular cluster M92. The variable star population of the cluster is studied with the aim of revising their classifications, identifications, frequency spectra and to select indicators of the parental cluster metallicity and distance. The Fourier decomposition of RR Lyrae light curves lead to the estimation of mean [Fe/H] spec = −2.20 ± 0.18, and distance of 8.3 ± 0.2 kpc. Four new variables are reported; one RRd (V40), a multimode SX Phe (V41), a SR (V42) and one RRc (F1) that is most likely not a cluster member. The AC nature of V7 is confirmed. The double mode nature of the RRc star V11 is not confirmed and its amplitude modulations are most likely due to the Blazhko effect. Two modes are found in the known RRc variable V13. It is argued that the variable V30, previously classified as RRab is, in fact, an BL Her-type star not belonging to the cluster. Making use of the Gaia-DR2 proper motions, we identified 5012 stars in the field of the cluster, that are very likely cluster members, and for which we possess photometry, enabling the production of a refined Colour-Magnitude Diagram. This also allowed us to identify a few variable stars that do not belong to the cluster. The RR Lyrae pulsation modes on the HB are cleanly separated by the first overtone red edge, a common feature in all Oo II type clusters.

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NGC 1261: A Time-Series Vi Study of Its Variable Stars

Cited by 7 publications

References 43 publications

NGC 6712: the variable star population of a tidally disrupted globular cluster

NGC 6712: the variable star population of a tidally disrupted globular cluster

Study of UV-bright stellar populations in the globular cluster NGC 1261 using Astrosat

CCD VI time-series of the extremely metal-poor globular cluster M92: revisiting its variable star population

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