A Census of the 32 Ori Association with Gaia*

Abstract

I have used high-precision photometry and astrometry from the third data release of Gaia (DR3) to identify candidate members of the 32 Ori association. Spectral types and radial velocities have been measured for subsets of the candidates using new and archival spectra. For the candidates that have radial velocity measurements, I have used UVW velocities to further constrain their membership, arriving at a final catalog of 169 candidates. I estimate that the completeness of the survey is ∼90% for spectral types of ≲M7 (≳0.06 M ⊙). The histogram of spectral types for the 32 Ori candidates exhibits a maximum at M5 (∼0.15 M ⊙), resembling the distributions measured for other young clusters and associations in the solar neighborhood. The available UVW velocities indicate that the association is expanding, but they do not produce a well-defined kinematic age. Based on their sequences of low-mass stars in color–magnitude diagrams, the 32 Ori association and Upper Centaurus-Lupus/Lower Centaurus-Crux (UCL/LCC) are coeval to within ±1.2 Myr, and they are younger than the β Pic moving group by ∼3 Myr, which agrees with results from previous analysis based on the second data release of Gaia. Finally, I have used mid-IR photometry from the Wide-field Infrared Survey Explorer to check for excess emission from circumstellar disks among the 32 Ori candidates. Disks are detected for 18 candidates, half of which are reported for the first time in this work. The fraction of candidates at ≤M6 that have full, transitional, or evolved disks is 10 / 149 = 0.07 − 0.02 + 0.03 , which is consistent with the value for UCL/LCC.

A Census of the Taurus Star-forming Region and Neighboring Associations with Gaia*

“…The histograms for the remaining associations near Taurus are plotted in Figure 10. The completeness limits are marked, which were derived in the manner described by Luhman (2022c). All of the associations exhibit a prominent peak near M5 (∼0.15 M e ), resembling the distributions in Taurus (Esplin & Luhman 2019) and other nearby associations and star-forming regions (e.g., Luhman 2022a).…”

“…The 32 Ori association was discovered by Mamajek (2007) and surveyed by Bell et al (2017). My catalog of candidates for 32 Ori from Gaia DR3 is presented in Luhman (2022c). Catalogs for the other seven associations are presented in this work.…”

“…Thus, the radial velocities and corresponding UVW velocities of such stars should be treated with caution. The radial velocities adopted from the LAMOST Medium-resolution Survey were derived from data at multiple epochs in the manner described by Luhman (2022c).…”

“…As with 93 Tau, I have named the five remaining associations after their brightest members. In Luhman (2022c) and Section 6, I compare my samples of candidates for the eight associations to those from previous studies.…”

“…A compilation of data for the candidate members of 32 Ori is provided in Luhman (2022c). The candidates for the remaining seven associations near Taurus are presented in Table 4, which includes source names from Gaia DR3 and previous studies; equatorial coordinates, proper motion, parallax, RUWE, and photometry from Gaia DR3; measurements of spectral types and the type adopted in this work; distance estimate based on the Gaia DR3 parallax (Bailer-Jones et al 2021); the most accurate available radial velocity measurement that has an error less than 4 km s −1 ; VSCATTER from APOGEE-2; UVW velocities; the designations and angular separations of the closest sources within 3″ from 2MASS and WISE; flags indicating whether the Gaia source is the closest match in DR3 for the 2MASS and WISE sources; photometry from 2MASS and WISE; flags indicating whether excesses are detected in three WISE bands and a disk classification if excess emission is detected (Section 5.4); and the name of the association to which the candidate is assigned.…”