Beyond Spectroscopy. I. Metallicities, Distances, and Age Estimates for Over 20 Million Stars from SMSS DR2 and Gaia EDR3

Abstract

Accurate determinations of stellar parameters and distances for large complete samples of stars are keys for conducting detailed studies of the formation and evolution of our Galaxy. Here we present stellar atmospheric parameters (effective temperature, luminosity classifications, and metallicity) estimates for some 24 million stars determined from the stellar colors of SMSS DR2 and Gaia EDR3, based on training data sets with available spectroscopic measurements from previous high/medium/low-resolution spectroscopic surveys. The number of stars with photometric-metallicity estimates is 4–5 times larger than that collected by the current largest spectroscopic survey to date—LAMOST—over the course of the past decade. External checks indicate that the precision of the photometric-metallicity estimates are quite high, comparable to or slightly better than that derived from spectroscopy, with typical values around 0.05–0.15 dex for both dwarf and giant stars with [Fe/H] > −2.01.0, 0.10–0.20 dex for giant stars with −2.0 < [Fe/H] ≤ −1.0, and 0.20–0.25 dex for giant stars with [Fe/H] ≤ −2.0, and include estimates for stars as metal-poor as [Fe/H] ∼ −3.5, substantially lower than previous photometric techniques. Photometric-metallicity estimates are obtained for an unprecedented number of metal-poor stars, including a total of over three million metal-poor (MP; [Fe/H] ≤ −1.0) stars, over half a million very metal-poor (VMP; [Fe/H] ≤ −2.0) stars, and over 25,000 extremely metal-poor (EMP; [Fe/H] ≤ −3.0) stars. Moreover, distances are determined for over 20 million stars in our sample. For the over 18 million sample stars with accurate Gaia parallaxes, stellar ages are estimated by comparing with theoretical isochrones. Astrometric information is provided for the stars in our catalog, along with radial velocities for ∼10% of our sample stars, taken from completed/ongoing large-scale spectroscopic surveys.

Dynamically Tagged Groups of Metal-poor Stars. II. The Radial Velocity Experiment Data Release 6

“…The biweight estimators of location and scale for the metallicity residuals determined from the medium-resolution spectra and the photometric metallicity yield μ = +0.14 dex and σ = 0.33 dex. As noted by Huang et al (2022), stars that have enhanced carbon often estimated photometric metallicities that are somewhat higher than the spectroscopic determinations, due to molecular carbon features affecting the blue narrow/medium-band filters v and u from SMSS (particularly for cooler carbon-enhanced stars). Stars with [C/Fe] c > +0.7, which we define as carbon-enhanced metal-poor (CEMP) stars are indicated with red circles around the dots shown in the bottom panel of Figure 4.…”

“…Of the 322,367 unique stars in RAVE DR6 that have acceptable quality-flags from the MADERA stellar parameter pipeline (Steinmetz et al 2020b), photometric metallicity and temperature estimates are taken from the SkyMapper Southern Survey (SMSS; Wolf et al 2018) Data Release 2 (DR2; Onken et al 2019). These estimates are derived by the procedure described in Huang et al (2022) and explained below. Metalpoor candidate stars from RAVE are also taken from the sample in Placco et al (2018) whose authors explored the nature of the RAVE stellar parameters with a corresponding set of spectroscopic parameters, also explained below.…”

“…Note that the elemental abundances reported here supersede those published in Placco et al (2018). 8 The rest of our sample is assembled from recent photometric estimates of temperature, luminosity classes, and metallicity for candidate stars from the RAVE DR6 survey based on the procedure described by Huang et al (2022). This study made use of recalibrated zero-points (Huang et al 2021a) We refer to these stars as the Full Sample.…”

“…The spectroscopic portion, [Fe/H] Spec , is derived purely from the spectroscopic follow-up, which showed that the RAVE pipeline is not entirely accurate in determining metallicity, hence the values also below [Fe/H] Spec = −1.5 in the spectroscopic sample. Based on these behaviors, we decided to use the photometric determinations based on the recalibrated SMSS DR2 from Huang et al (2022) for the present analysis, rather than the RAVE DR6 estimates.…”

The Complexity of the Cetus Stream Unveiled from the Fusion of STREAMFINDER and StarGO

“…On the other hand, the northern MDF has a bimodal distribution, which clearly shows a large source of metal-rich stars that are likely contaminants (see the discussion in Section 2.3). Similarly, using the photometric metallicities derived from SkyMapper DR2 data (Huang et al 2021(Huang et al , 2022, the northern set shows an extended distribution, and the southern MDF exhibits a tight distribution with an average [Fe/H] = −2.3 shown as blue histograms. There are 35 out of 41 stars that are VMP in the southern set.…”