Physical parameters of three field RR Lyrae stars

This work was partially supported by DGAPA–Universidad Nacional Autonoma de Mexico through project IN104612.Stromgren uvby - beta photometry of the stars classified as RR Lyrae stars RU Piscium, SS Piscium and TU Ursae Majoris has been used to estimate their iron abundance, temperature, gravity and absolute magnitude. The stability of the pulsating period is discussed. The nature of SS Psc as a RRc or a HADS is addressed. The reddening of each star is estimated from the Stromgren colour indices and reddening sky maps. The results of three approaches to the determination of [Fe/H], T-eff and log(g) are discussed: Fourier light curve decomposition, the Preston Delta S index and the theoretical grids on the (b - y)(o) - c(10) plane.Peer reviewe

Many new variable stars discovered in the core of the globular cluster NGC 6715 (M 54) with EMCCD observations

Context. We show the benefits of using electron-multiplying CCDs and the shift-and-add technique as a tool to minimise the effects of atmospheric turbulence, such as blending between stars in crowded fields, and to avoid saturated stars in the fields observed. We intend to complete, or improve on, the census of the variable star population in globular cluster NGC 6715. Aims. Our aim is to obtain high-precision time-series photometry of the very crowded central region of this stellar system via the collection of better angular resolution images than has been previously achieved with conventional CCDs on ground-based telescopes. Methods. Observations were carried out using the Danish 1.54-m telescope at the ESO La Silla observatory in Chile. The telescope is equipped with an electron-multiplying CCD that enables short-exposure-time images to be obtained (ten images per second) that were stacked using the shift-and-add technique to produce the normal-exposure-time images (minutes). The high precision photometry was performed via difference image analysis employing the DanDIA pipeline. We attempted automatic detection of variable stars in the field. Results. We statistically analysed the light curves of 1405 stars in the crowded central region of NGC 6715 to automatically identify the variable stars present in this cluster. We found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 irregular (long-period type), 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). Photometric measurements for these stars are available in electronic form through the Strasbourg Astronomical Data Centre

Variable stars in the globular cluster NGC 7492. New discoveries and physical parameters determination

We have performed a photometric V, R, I CCD time-series analysis with a baseline of ~8 years of the outer-halo globular cluster NGC 7492 with the aim of searching for new variables and using these (and the previously known variables) to determine the physical parameters of interest for the cluster (e.g. metallicity, absolute magnitude of the horizontal branch, distance, etc.). We use difference image analysis (DIA) to extract precise light curves in the relatively crowded star field, especially towards the densely populated central region. Several approaches are used for variability detection that recover the known variables and lead to new discoveries. We determine the physical parameters of the only RR0 star using light curve Fourier decomposition analysis. We find one new long period variable and two SX Phe stars in the blue straggler region. We also present one candidate SX Phe star which requires follow-up observations. Assuming that the SX Phe stars are cluster members and using the period-luminosity relation for these stars, we estimate their distances as ~25.2+-1.8 and 26.8+-1.8 kpc, and identify their possible modes of oscillation. We refine the periods of the two RR Lyrae stars in our field of view. We find that the RR1 star V2 is undergoing a period change and possibly exhibits the Blazhko effect. Fourier decomposition of the light curve of the RR0 star V1 allows us to estimate the metallicity [Fe/H]_ZW-1.68+-0.10 or [Fe/H]_UVES-1.64+-0.13, log-luminosity ~1.76+-0.02, absolute magnitude ~0.38+-0.04 mag, and true distance modulus of ~16.93+-0.04 mag, which is equivalent to a distance of ~24.3+-0.5 kpc. All of these values are consistent with previous estimates in the literature.Comment: 12 pages, 13 figures, 6 tables, accepted for publication in A&

RR Lyrae mode switching in globular cluster M 68 (NGC 4590)

D.M.B. acknowledges NPRP grant # X-019-1-006 from the Qatar National Research Fund (a member of Qatar Foundation). A.A.F. acknowledges the support of DGAPA-UNAM through project IN106615-17.We build on our detailed analysis of time-series observations of the globular cluster M 68 to investigate the irregular pulsational behaviour of four of the RR Lyrae stars in this cluster. M 68 is one of only two globular clusters in which mode switching of RR Lyrae stars has previously been reported. We discuss one additional case, as well as a case of irregular behaviour, and we briefly revisit the two previously reported cases with a homogeneous analysis. We find that in 2013, V45 was pulsating in the first-overtone mode alone, despite being previously reported as a double-mode (fundamental and first overtone) pulsator in 1994, and that the amplitude of the fundamental mode in V7 is increasing with time. We also suggest that V21 might not have switched pulsation modes as previously reported, although the first overtone seems to be becoming less dominant. Finally, our analysis of available archival data confirms that V33 lost a pulsation mode between 1950 and 1986.Publisher PDFPeer reviewe

Variable stars in the bulge globular cluster NGC 6401

We present a study of variable stars in globular cluster NGC 6401. The cluster is only 5.3º away from the Galactic centre and suffers from strong differential reddening. The photometric precision afforded us by difference image analysis resulted in improved sensitivity to variability in formerly inaccessible interior regions of the cluster. We find 23 RRab and 11 RRc stars within one cluster radius (2.4'), for which we provide coordinates, finder-charts and time-series photometry. Through Fourier decomposition of the RR Lyrae star light curves we derive a mean metallicity of [Fe/H]UVES = -1.13 ± 0.06 ([Fe/H]ZW = -1.25 ± 0.06), and a distance of d ≈ 6.35 ± 0.81 kpc. Using the RR Lyrae population, we also determine that NGC 6401 is an Oosterhoff type I cluster.PostprintPeer reviewe

Investigation of variable star candidates in the globular cluster NGC 5024 (M53)

We have performed a careful investigation of the 74 candidate variable stars presented by Safanova & Stalin (2011). For this purpose we used our data base of imaging and light curves from Arellano Ferro et al. (2011) and Arellano Ferro et al. (2012). We find that two candidates are known variable stars, eight candidates were discovered first by Arellano Ferro et al. (2011) but would not have been known to Safanova & Stalin (2011) at the time of their paper submission, while four candidates are new variables. Three of the new variables are SX Phe type and one is a semi-regular red giant variable (SR type). We also tentatively confirm the presence of true variability in two other candidates and we are unable to investigate another four candidates because they are not in our data base. However, we find that the remaining 54 candidate variable stars are spurious detections where systematic trends in the light curves have been mistaken for true variability. We believe that the erroneous detections are caused by the adoption of a very low detection threshold used to identify these candidates.Comment: MNRAS accepte