Spectro-photometric distances to stars: A general purpose Bayesian approach

Abstract

Context. Determining distances to individual field stars is a necessary step towards mapping Galactic structure and determining spatial variations in the chemo-dynamical properties of stellar populations in the Milky Way. Aims. In order to provide stellar distance estimates for various spectroscopic surveys, we have developed a code that estimates distances to stars using measured spectroscopic and photometric quantities. We employ a Bayesian approach to build the probability distribution function over stellar evolutionary models given these data, delivering estimates of model parameters (including distances) for each star individually. Our method provides several alternative distance estimates for each star in the output, along with their associated uncertainties. This facilitates the use of our method even in the absence of some measurements. Methods. The code was first tested on simulations, successfully recovering input distances to mock stars with 1% bias. We found the uncertainties scale with the uncertainties in the adopted spectro-photometric parameters. The method-intrinsic random distance uncertainties for typical spectroscopic survey measurements amount to around 10% for dwarf stars and 20% for giants, and are most sensitive to the quality of log g measurements. Results. The code was then validated by comparing our distance estimates to parallax measurements from the H mission for nearby stars (<300 pc), to asteroseismic distances of CoRoT red giant stars, and to known distances of well-studied open and globular clusters. The photometric data of these reference samples cover both optical and infrared wavelengths. The spectroscopic parameters are also based on spectra taken at various wavelengths, with varying spectral coverage and resolution: the Sloan Digital Sky Survey programs SEGUE and APOGEE, as well as various ESO instruments. Conclusions. External comparisons confirm that our distances are subject to very small systematic biases with respect to the fundamental H scale (+0.4% for dwarfs, and +1.6% for giants). The typical random distance scatter is 18% for dwarfs, and 26% for giants. For the CoRoT-APOGEE sample, which spans Galactocentric distances of 4−14 kpc, the typical random distance scatter is 15% both for the nearby and farther data. Our distances are systematically larger than the CoRoT distances by about +9%, which can mostly be attributed to the different choice of priors. The comparison to known distances of star clusters from SEGUE and APOGEE has led to significant systematic differences for many cluster stars, but with opposite signs and substantial scatter. Finally, we tested our distances against those previously determined for a high-quality sample of giant stars from the RAVE survey, again finding a small systematic trend of +5% and an rms scatter of 30%. Efforts are underway to provide our code to the community by running it on a public server.

Chemical trends in the Galactic halo from APOGEE data

“…All produce fairly similar results, but we choose to employ the BPG estimates derived from the peak of the second probability distribution function in their analysis (BPG2p -see Santiago et al 2016 Figure 1) are around ∼ −1.5, which is consistent with the previous works. As mentioned above in Section 3.1, we have derived the MDF for our sample from the calibrated [M/H], shown in the top panel of Figure 1.…”

“…A number of independent groups have been working on the derivation of distance estimates for APOGEE stars, which we consider in our present analysis; these are described in Santiago et al (2016), Hayden et al (2015), and Schultheis et al (2014).…”

Disentangling the Galactic Halo with APOGEE. II. Chemical and Star Formation Histories for the Two Distinct Populations

“…This agrees with our results indicating that HMg stars were formed in a stronger star formation event, with a shorter decline time than LHg ones. Figure 11 shows the space distribution of our sample, using distances from the BPG (Santiago et al 2016). From the xy, yz, and xz planes (in Galactic coordinates), we see that the HMg population is mainly confined at inner regions of the halo.…”

“…Thus, to explore stars with halo kinematics we select objects with v rad >180 km s We exclude stars with distances (adopted using the techniques of the Brazilian Participation Group-BPG; Santiago et al 2016) from the Galactic center  r 4 kpc, in order to avoid bulge stars. We know that this selection cut may also exclude some halo stars located at this range of distances.…”

The Apache Point Observatory Galactic Evolution Experiment (APOGEE)

“…This management scheme gives rise to the brightness modulation pattern apparent in this image. 74 Distances are calculated using the method of Santiago et al (2016) , and metallicity errors less than 0.3 dex. The distance inference uses PARSEC isochrones (Bressan et al 2012) applied to the spectro-photometric data from APOGEE and 2MASS with the overall metallicity of each star determined as the [ ] Fe H measured from iron lines plus the overall a [ ] Fe abundance determined from the synthetic fit to the whole APOGEE spectrum (see Santiago et al 2016 for further details).…”