Distances to Galactic OB Stars: Photometry versus Parallax

Abstract

For application to surveys of interstellar matter and Galactic structure, we compute new spectrophotometric distances to 139 OB stars frequently used as background targets for UV spectroscopy. Many of these stars have updated spectral types and digital photometry with reddening corrections from the Galactic O-Star (GOS) spectroscopic survey. We compare our new photometric distances to values used in previous IUE and FUSE surveys and to parallax distances derived from Gaia-DR2, after applying a standard (0.03 mas) offset from the quasar celestial reference frame. We find substantial differences between photometric and parallax distances at d > 1.5 kpc, with increasing dispersion when parallax errors exceed 8%. Differences from previous surveys arise from new GOS stellar classifications, especially luminosity classes, and from reddening corrections. We apply our methods to two OB associations. For Perseus OB1 (nine O-stars) we find mean distances of 2.47 ± 0.57 kpc (Gaia parallax) and 2.99 ± 0.14 kpc (photometric) using a standard grid of absolute magnitudes . For 29 O-stars in Car OB1 associated with Trumpler-16, Trumpler-14, Trumpler-15, and Collinder-228 star clusters, we find 2.87 ± 0.73 kpc (Gaia) and 2.60 ± 0.28 kpc (photometric). Using an alternative grid of O-star absolute magnitudes (Martins et al. 2005) shifts these photometric distances ∼7% closer. Improving the distances to OB-stars will require attention to spectral types, photometry, reddening, binarity, and the grid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3 will improve the precision of distances to massive star-forming regions in the Milky Way.

Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – II. Cluster and association membership

“…Indeed, from an inspection of v3 of the Galactic O star Catalogue (Maíz Apellániz et al 2013), only 42% of O stars are thought to be members of star clusters, with almost three quarters located in OB associations and/or low density (<100s stars pc −3 ) star forming regions (as summarised in Table 1). These statistics are likely to be upper limits given membership has not been confirmed from Gaia proper motions/parallaxes, although some comparisons with earlier distance estimates have been undertaken (Shull & Danforth 2019).…”

“…WR24 is also a possible member of Collinder 228, although this is difficult to confirm, as the cluster contains stars exhibiting a wide range of proper motions Molina-Lera et al (2016). investigate the complex structure of Carina, identifying a foreground population at 1.4-2.3 kpc (corresponding to Trumpler 18), a second population distributed over 2.0-3.3 kpc, plus a background group Shull & Danforth (2019). obtain 2.87 ± 0.73 kpc 2 for 29 O star members of Trumpler 14-16 and Collinder 228 based on…”

Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – I. Distances and absolute magnitudes

“…Indeed, from an inspection of v3 of the Galactic O star Catalogue (Maíz Apellániz et al 2013), only 42% of O stars are thought to be members of star clusters, with almost three quarters located in OB associations and/or low density (<100s stars pc −3 ) star forming regions (as summarised in Table 1). These statistics are likely to be upper limits given membership has not been confirmed from Gaia proper motions/parallaxes, although some comparisons with earlier distance estimates have been undertaken (Shull & Danforth 2019). For optically visible WR stars, Lundström & Stenholm (1984) found that only 10-30% of stars identified at that time lay within clusters, which was updated by van der Hucht (2001) to include WR stars identified from infrared surveys, revealing that 35% of Galactic WR stars were thought to be located either in clusters or associations.…”

Similar but different: the varied landscape of Onfp/Oef stars variability

“…which is usually classified as a supergiant. Whilst GAIA-DR2 parallaxes of stars brighter than sixth magnitude must be considered with caution, we note that the GAIA parallax of λ Cep was determined at the 12.7 σ level and indicates a distance around 0.61 kpc (Shull & Danforth 2019). This is significantly lower than the usually adopted distance of 0.7 -1.0 kpc (see Sudnik & Henrichs 2016;Shull & Danforth 2019).…”

“…Whilst GAIA-DR2 parallaxes of stars brighter than sixth magnitude must be considered with caution, we note that the GAIA parallax of λ Cep was determined at the 12.7 σ level and indicates a distance around 0.61 kpc (Shull & Danforth 2019). This is significantly lower than the usually adopted distance of 0.7 -1.0 kpc (see Sudnik & Henrichs 2016;Shull & Danforth 2019). Yet, the values of log g = 3.50 ± 0.15, 3.39 ± 0.06 or 3.55 ± 0.10 inferred by Martins et al (2015); Cazorla et al (2017), Holgado et al (2020) and Repolust et al (2004) from model atmosphere fitting favour a supergiant luminosity class (log g = 3.48 for an O6 I, Martins et al 2005), although the giant luminosity class (log g = 3.65 for an O6 III star, Martins et al 2005) lies within the errors of most of the above log g values.…”