The VMC survey - V. First results for classical Cepheids

Abstract

The VISTA Magellanic Cloud (VMC; PI: M.‐R. L. Cioni) survey is collecting deep Ks‐band time series photometry of the pulsating variable stars hosted by the system formed by the two Magellanic Clouds (MCs) and the bridge connecting them. In this paper, we present the first results for classical Cepheids, from the VMC observations of two fields in the Large Magellanic Cloud (LMC), centred on the South Ecliptic Pole and the 30 Doradus star‐forming regions, respectively. The VMC Ks‐band light curves of the Cepheids are well sampled (12 epochs) and of excellent precision (typical errors of ∼0.01 mag). We were able to measure for the first time the Ks magnitude of the faintest classical Cepheids in the LMC (Ks ∼ 17.5 mag), which are mostly pulsating in the first overtone (FO) mode, and to obtain FO period–luminosity (PL), period–Wesenheit (PW) and period–luminosity–colour (PLC) relations, spanning the full period range from 0.25 to 6 d. Since the longest period Cepheid in our data set has a variability period of 23 d, we have complemented our sample with literature data for brighter F Cepheids. On this basis, we have built a PL relation in the Ks band that, for the first time, includes short‐period – hence low‐luminosity – pulsators, and spans the full range from 1.6 to 100 d in period.We also provide the first ever empirical PW and PLC relations using the (V − Ks) colour and time series Ks photometry. The very small dispersion (∼0.07 mag) of these relations makes them very well suited to study the three‐dimensional geometry of the Magellanic system. The use of ‘direct’ (parallax‐ and Baade–Wesselink‐based) distance measurements to both Galactic and LMC Cepheids allowed us to calibrate the zero‐points of the PL, PW and PLC relations obtained in this paper, and in turn to estimate an absolute distance modulus of (m − M)0 = 18.46 ± 0.03 mag for the LMC. This result is in agreement with most of the latest literature determinations based on classical Cepheids.

The VMC Survey – XIII. Type II Cepheids in the Large Magellanic Cloud★

“…An inspection of the table reveals that the DM0,LMC calculated by means of CCs (column 2 in Table 8) and by means of the T2CEPs differ by more than ∼0.1 mag, even if, formally there is agreement within 1 σ. Since both the Ripepi et al (2012b) calibration for CCs and that presented here for T2CEPs are based on a weighted mix of HST parallaxes and BW analysis, this discrepancy, albeit only partially significant, seems to suggest that the distance scales calibrated on pulsating stars belonging to population I and population Table 7. Values for the coefficients of the P L, P W and P LC relations for BL Her and W Vir Cepheids taken from the literature.…”

“…ical results if the quoted relationships are calibrated with the Galactic T2CEPs and RR lyrae, whereas there is a ∼0.1 mag discrepancy if we adopt the CC-based DM by Ripepi et al (2012b) for the LMC to define the ZP . However, if we take into account the uncertainties this discrepancy results formally not significant within 1σ.…”

“…11. For comparison, column (2) of Table 6 shows the ZP s obtained assuming DM0,LMC = 18.46 ± 0.03 mag, as derived by Ripepi et al (2012b) from LMC CC stars. We choose the work by Ripepi et al (2012b) as reference for CCs because: i) these Authors adopted a procedure similar to the one adopted in this work; ii) their results are in excellent agreement with the most recent and accurate literature findings (see e.g.…”

“…For comparison, column (2) of Table 6 shows the ZP s obtained assuming DM0,LMC = 18.46 ± 0.03 mag, as derived by Ripepi et al (2012b) from LMC CC stars. We choose the work by Ripepi et al (2012b) as reference for CCs because: i) these Authors adopted a procedure similar to the one adopted in this work; ii) their results are in excellent agreement with the most recent and accurate literature findings (see e.g. Storm et al 2011b;Laney, Joner, & Pietrzyński 2012;Walker 2012;Pietrzyński et al 2013;de Grijs, Wicker, & Bono 2014, and references therein) An analysis of Table 6 reveals that: i) the inclusion of the two stars with BW-based distances does not change significantly the ZP s and ii) there is a difference of at least ∼0.1 mag between the ZP s calibrated on the basis of CCs and of Galactic T2CEPs (see Sect.…”

“…Table 6. P L(Ks) and P W (Ks) relations for LMC T2CEPs with the ZP calibrated as follows: (2) by imposing a DM LM C,0 =18.46±0.03 mag (from CCs in the LMC Ripepi et al 2012b) in Table 5; (3) by adopting Galactic T2CEP (κ Pav) and RR Lyrae variables with HST parallaxes (Benedict et al 2011) and T2CEPs with BW distance estimates (Feast et al 2008); (4) …”