New type II Cepheids from VVV data towards the Galactic center

Abstract

Context. The Galactic center (GC) is the densest region of the Milky Way. Variability surveys towards the GC potentially provide the largest number of variable stars per square degree within the Galaxy. However, high stellar density is also a drawback due to blending. Moreover, the GC is affected by extreme reddening, therefore near infrared observations are needed. Aims. We plan to detect new variable stars towards the GC, focusing on type II Cepheids (T2Cs) which have the advantage of being brighter than RR Lyrae stars. Methods. We perform parallel Lomb-Scargle and Generalized Lomb-Scargle periodogram analysis of the Ks-band time series of the VISTA variables in the Vía Láctea survey, to detect periodicities. We employ statistical parameters to clean our sample. We take account of periods, light amplitudes, distances, and proper motions to provide a classification of the candidate variables. Results. We detected 1,019 periodic variable stars, of which 164 are T2Cs, 210 are Miras and 3 are classical Cepheids. We also found the first anomalous Cepheid in this region. We compare their photometric properties with overlapping catalogs and discuss their properties on the color-magnitude and Bailey diagrams. Conclusions. We present the most extensive catalog of T2Cs in the GC region to date. Offsets in E(J − Ks) and in the reddening law cause very large (∼1-2 kpc) uncertainties on distances in this region. We provide a catalog which will be the starting point for future spectroscopic surveys in the innermost regions of the Galaxy. 1,000 were found in the Bulge by OGLE, while the same survey detected more than 38,000 RRLs Soszyński et al. 2014Soszyński et al. , 2017. On the other hand, T2Cs are brighter than RRLs. Moreover, the majority of the T2Cs are old (>10 Gyr) stars, although it was recently suggested that the W Vir (WV) subclass might partly be associated to the intermediate-age population (Iwanek et al. 2018). This means that they can also be found in stellar systems with no recent star formation events, which is a requirement to find CCs as they are purely young (<400 Myr, Bono et al. 2005; Anderson et al. 2016) stars. For a recent review of the properties of T2Cs, see the monography by Catelan & Smith (2015).The aim of this paper is to find new variables in the central-most tile of the VVV survey, focusing on the detection of T2Cs. The paper is organized as follows: in Section 2, we present the data. In Section 3, we describe the light-curve analysis, including the variable star search, period determinations, and light-curve fittings. In Section 4, we discuss the classification of periodic variable stars using all tools available. We present the matches with other existing catalogs of GC variable stars in Section 5. In Section 6, we discuss the position, color-magnitude diagrams (CMDs) and Bailey (amplitude vs. period) diagrams of the final list of variables, and the obtained distances to T2Cs. Finally, the conclusions are presented in Section 7. DataPhotometry -We used proprietary PSF photometric reduction )...

On the optimal calibration of VVV photometry

“…Particularly in the most crowded VVV fields, PSF photometry has already proven its great value in generating deep color-magnitude diagrams and enabling time-series studies, even of faint sources (e.g., [2,5,9]). For PSF photometry, the light-curve scatter caused by the varying offsets between individual epochs (see the difference in scatter between corrected and uncorrected light-curves in Fig.…”

Evolutionary and pulsation properties of Type II Cepheids

“…In spite of the crucial role that type II Cepheids (TIICs 1 played in the cosmic distance scale (Baade 1958;Bono et al 2016) and the ongoing observational effort for the identification in the Galactic center (Matsunaga et al 2013;Braga et al 2019), in the Galactic Bulge (Soszyński et al 2011(Soszyński et al , 2017, in Galactic clusters (Matsunaga et al 2006) and in the Magellanic Clouds (Soszyński et al 2018) the investigations concerning their evolutionary and pulsation properties lag when compared with other groups of radial variables. According to their pulsation properties (period distribution, shape of the light curve) TIICs can be classified into three different sub-groups (Soszyński et al 2008): BL Herculis (BLH) have periods longer than RRLs (P 1 day) and shorter than five days, W Virginis (WVs) have periods between five and twenty days, while RV Tauri (RVTs) have periods longer than twenty days (see Fig.…”

The VVV Survey: Globular Clusters and more

“…We initially discovered a dozen RR Lyrae in the Galactic center (Minniti et al (2016)). A follow-up search revealed about a thousand RR Lyrae in the region within R G = 1.6 deg from the Galactic center (Contreras Ramos et al (2018b)), as well as dozens of Type II Cepheids (Braga et al (2019)). This confirms that the innermost part of the Galaxy contains a non-negligible old and metal-poor component.…”