The Long-lived Double-peaked Hα Emission of VES 735: A Herbig Oe Star?

Hα is known to be a tracer for circumstellar disks around both T Tauri and Herbig Ae/Be stars (Herbig 1962). We report the long-lived, double-peaked Hα emission from the high-mass O star VES 735. VES 735 was initially cataloged in a survey for Hα emission objects in the Milky Way (Coyne & MacConnell 1983). Kerton et al. (1999) determined that it is the exciting source for the H ii region/bubble KR 140. They established an atlas of massive O stars of different spectral types between 4800—5420 Å and determined that the spectral type of VES 735 is approximately O8.5 V. The second data release (DR2) from Gaia (Gaia Collaboration et al. 2018) gives a parallax of 0.6776 ± 0.0537 mas, or distance of 1.47 ± 0.12 kpc. With this new distance, we use the spectral type determination technique from Watson et al. (2008), which matches the observed spectral energy distribution with model stellar atmospheres (Kurucz 1993). Using this method we find a spectral type between O8 V and O8.5 V, in good agreement with Kerton et al. (1999).

Observations from the Dominion Astrophysical Observatory and David Dunlap Observatory in 1996 and 1997 showed that VES 735 displays double-peaked Hα emission (Figure 1, left). This trait is more commonly found in Be type stars and is likely caused by emission from a rotating disk of circumstellar material (Conti & Leep 1974). This emission is similar to what is seen in the well known example of an Oe star, ζ Oph (Niemelä & Méndez 1974). However, the Hα emission from ζ Oph is sporadic, with outbursts of emission occurring for only a few months at a time and years elapsing between episodes. The most common model to explain this phenomenon is that the rapidly rotating star undergoes mass loss by some mechanism and ejects material into the circumstellar environment. Winds from the star eventually disperse the orbiting material causing the emission features to fade and disappear (Ebbets 1981). In contrast, VES 735 was found to display continuous, double-peaked emission and thus the emission is not likely a due to a mass-loss event. It was determined that the Hα peaks were separated by $v\,\sin \,i\simeq 400$ km s⁻¹, with some variability of the peak separation and strength between observations.

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More recent observations of VES 735 taken at the Wyoming Infrared Observatory show that it still exhibits similar double-peaked Hα emission in 2013 and 2014, 18 yr after this feature was first observed. The emission still shows slight variability, but not a simple decrease with time, as the strongest emission of the second set of observations is seen in 2014 August (Figure 1, right). VES 735 was also observed in the K-band during 2016 October with the Gemini Near-InfraRed Spectrograph (Elias et al. 2006) (program ID: GN-2016-FT-17), where it displayed Brγ emission, a high density indicator, which may originate in an ionized disk wind (Bik et al. 2005).

Given the long-lived nature of the emission, and the association of the star with a fairly young H ii region, an alternative to the traditional Oe model is that VES 735 is a higher-mass equivalent of a Herbig Be star. In this case the material surrounding the star would be a remnant accretion disk (e.g., Hollenbach et al. 1994) rather than material associated with mass loss.

VES 735 was first identified as having Hα emission over 35 yr ago, and it has exhibited continued hydrogen emission at least through 2016, as seen in Brγ observations. Double-peaked Hα emission is likely continuous on decadal timescales based on observations between 1996 and 2014. This makes VES 735 an ideal candidate for continued spectroscopic monitoring to examine the longevity and variability of disks, remnant or otherwise, around high-mass O stars.