Carbon Stars from Gaia Data Release 3 and the Space Density of Dwarf Carbon Stars

Abstract Carbon stars (with atmospheric C/O &gt; 1) range widely in temperature and luminosity, from low-mass dwarfs to asymptotic giant branch (AGB) stars. The main-sequence dwarf carbon (dC) stars have inherited carbon-rich material from an AGB companion, which has since transitioned to a white dwarf. The dC stars are far more common than C giants, but no reliable estimates of the dC space density have been published to date. We present results from an all-sky survey for carbon stars using the low-resolution XP spectra from Gaia Data Release 3. We developed and measured a set of spectral indices contrasting C 2 and CN molecular band strengths in carbon stars against common absorption features found in normal (C/O &lt; 1) stars, such as Ca i , TiO, and Balmer lines. We combined these indices with the XP spectral coefficients as inputs to supervised machine learning algorithms trained on a vetted sample of known C stars from LAMOST. We describe the selection of the carbon candidate sample and provide a catalog of 43,574 candidates dominated by cool C giants in the Magellanic Clouds and at low Galactic latitude in the Milky Way. We report the confirmation of candidate C stars using intermediate-resolution ( R ∼ 1800) optical spectroscopy from the Fred Lawrence Whipple Observatory and provide estimates of the sample purity and completeness. From a carefully vetted sample of over 600 dCs, we measure their local space density to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:mo>=</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>1.9</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.14</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>pc</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> (about one dC in every local disk volume of radius 50 pc), with a relatively large disk scale height of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>z</mml:mi> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:mo>=</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>85</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>43</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>49</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> </mml:math> pc.