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Register a new userA near-infrared survey of Miras and the distance to the Galactic Centre
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Noriyuki Matsunaga
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We report the results of a near-infrared survey for long-period variables in a field of view of 20 arcmin by 30 arcmin towards the Galactic Centre (GC). We have detected 1364 variables, of which 348 are identified with those reported in Glass et al. (2001). We present a catalogue and photometric measurements for the detected variables and discuss their nature. We also establish a method for the simultaneous estimation of distances and extinctions using the period-luminosity relations for the JHKs bands. Our method is applicable to Miras with periods in the range 100--350 d and mean magnitudes available in two or more filter bands. While J-band means are often unavailable for our objects because of the large extinction, we estimated distances and extinctions for 143 Miras whose H- and Ks-band mean magnitudes are obtained. We find that most are located at the same distance to within our accuracy. Assuming that the barycentre of these Miras corresponds to the GC, we estimate its distance modulus to be 14.58+-0.02(stat.)+-0.11(syst.) mag, corresponding to 8.24+-0.08(stat.)+-0.42(syst.) kpc. We have assumed the distance modulus to the LMC to be 18.45 mag, and the uncertainty in this quantity is included in the systematic error above. We also discuss the large and highly variable extinction. Its value ranges from 1.5 mag to larger than 4 mag in A(Ks) except towards the thicker dark nebulae and it varies in a complicated way with the line of sight. We have identified mid-infrared counterparts in the Spitzer/IRAC catalogue of Ramirez et al. (2008) for most of our variables and find that they follow rather narrow period-luminosity relations in the 3.6 to 8.0 micrometre wavelength range.
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Mira variables are useful distance indicators, due to their high luminosities and well-defined period-luminosity relation. We select 1863 Miras from SAAO and MACHO observations to examine their use as distance estimators in the Milky Way. We measure a distance to the Galactic centre of $R_0 = 7.9 pm 0.3$ kpc, which is in good agreement with other literature values. The uncertainty has two components of $sim$0.2 kpc each: the first is from our analysis and predominantly due to interstellar extinction, the second is due to zero-point uncertainties extrinsic to our investigation, such as the distance to the Large Magellanic Cloud (LMC). In an attempt to improve existing period-luminosity calibrations, we use theoretical models of Miras to determine the dependence of the period-luminosity relation on age, metallicity, and helium abundance, under the assumption that Miras trace the bulk stellar population. We find that at a fixed period of $log P = 2.4$, changes in the predicted $K_s$ magnitudes can be approximated by $Delta M_{Ks} approx -0.109(Delta rm{[Fe/H]}) + 0.033( {Delta}t/rm{Gyr}) + 0.021 ({Delta}Y/0.01)$, and these coefficients are nearly independent of period. The expected overestimate in the Galactic centre distance from using an LMC-calibrated relation is $sim$0.3 kpc. This prediction is not validated by our analysis; a few possible reasons are discussed. We separately show that while the predicted color-color diagrams of solar-neighbourhood Miras work well in the near-infrared, though there are offsets from the model predictions in the optical and mid-infrared.
Due to the extreme extinction towards the Galactic centre ($A_{V} sim 30$ mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. We studied the dependence of the extinction index ($alpha_lambda$) in the NIR on the line of sight, wavelength, and extinction. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue ($0.2$) for the inner part of the Galaxy in $JHK_s$, and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. We did not detect any significant line-of-sight or extinction variation in $alpha$ within the studied region in the nuclear stellar disc. The extinction index between $JH$ and $HK_s$ differs by $0.19 pm 0.05$. We obtained mean values for the extinction indices $alpha_{JH} = 2.43pm0.03$ and $alpha_{HK_s} = 2.23pm0.03$. The dependence of the extinction index on the wavelength could explain the differences obtained for $alpha_lambda$ in the literature since it was assumed constant for the NIR regime.
We present the near-infrared observations of population II Cepheids in the Galactic bulge from VVV survey. We identify 340 population II Cepheids in the Galactic bulge from VVV survey based on their match with OGLE-III Catalogue. The single-epoch $JH$ and multi-epoch $K_s$ observations complement the accurate periods and optical $(VI)$ mean-magnitudes from OGLE. The sample consisting of BL Herculis and W Virginis subtypes is used to derive period-luminosity relations after correcting mean-magnitudes for the extinction. Our $K_s$-band period-luminosity relation, $K_s = -2.189(0.056)~[log(P) - 1] + 11.187(0.032)$, is consistent with published work for BL Herculis and W Virginis variables in the Large Magellanic Cloud. We present a combined OGLE-III and VVV catalogue with periods, classification, mean magnitudes and extinction for 264 Galactic bulge population II Cepheids having good-quality $K_s$-band light curves. The absolute magnitudes for population II Cepheids and RR Lyraes calibrated using Gaia and Hubble Space Telescope parallaxes, together with calibrated magnitudes for Large Magellanic Cloud population II Cepheids, are used to obtain a distance to the Galactic center, $R_0=8.34pm0.03{mathrm{(stat.)}}pm0.41{mathrm{(syst.)}}$, which changes by $^{+ 0.05}_{-0.25}$ with different extinction laws. While noting the limitation of small number statistics, we find that the present sample of population II Cepheids in the Galactic bulge shows a nearly spheroidal spatial distribution, similar to metal-poor RR Lyrae variables. We do not find evidence of the inclined bar as traced by the metal-rich red-clump stars. The number density for population II Cepheids is more limited as compared to abundant RR Lyraes but they are bright and exhibit a wide range in period that provides a robust period-luminosity relation for an accurate estimate of the distance to the Galactic center.
We study the near-infrared properties of 690 Mira candidates in the central region of the Large Magellanic Cloud, based on time-series observations at JHKs. We use densely-sampled I-band observations from the OGLE project to generate template light curves in the near infrared and derive robust mean magnitudes at those wavelengths. We obtain near-infrared Period-Luminosity relations for Oxygen-rich Miras with a scatter as low as 0.12 mag at Ks. We study the Period-Luminosity-Color relations and the color excesses of Carbon-rich Miras, which show evidence for a substantially different reddening law.
We present near-IR imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra ACIS-I survey towards the Galactic Centre (GC) (Wang et al. 2002). These ~800 discrete sources represent an important and previously undetected population within the Galaxy. From our VLT observations of 77 X-ray sources, we identify candidate K-band counterparts to 75% of the Chandra sources in our sample. The near-IR magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC.
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