[Paper Review] Optical Spectroscopy of V635 Cassiopeiae/4U 0115+63

S. J. Unger, P. F. Roche+4 more|arXiv (Cornell University)|Feb 7, 1998

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This paper presents optical spectroscopy of V635 Cassiopeiae/4U 0115+63, identifying it as an O9e star—marking the first direct spectral classification of this X-ray binary's optical counterpart. The study reveals extreme variability in Hα and Paschen lines, interpreted as a disk-loss event, and reclassifies the flux standard Hiltner 102 as O9.7 II with nitrogen enhancement.

ABSTRACT

V635 Cas is the optical counterpart of the X-ray binary system 4U 0115+63. It was previously tentatively identified as a Be star based on its optical colours and the presence of H alpha emission. Our observations indicate that it is an O9e star. This is the first direct determination of this star's optical spectral type. The presence of a hotter companion star may in part explain the large temporal variation observed in this system. Extreme variability was observed in 1992 February when both the H alpha and a series of Paschen lines changed from emission to absorption. This was interpreted as a disk-loss event and it is the first time that it has been observed in this system. We use far red spectra of V635 Cas to probe the circumstellar disk, discussing the various line formation regions. The lines observed are consistent with a late type Oe star. The flux standard Hiltner 102 was also observed. Although it is classified as a B0 III star, we re-classify it as a O9.7 II star with a slight nitrogen enhancement.

Motivation & Objective

• To determine the optical spectral type of V635 Cassiopeiae/4U 0115+63, the optical counterpart of an X-ray binary.
• To investigate the nature of extreme variability in Hα and Paschen lines observed in 1992.
• To analyze circumstellar disk structure through far-red spectroscopy of emission and absorption features.
• To re-evaluate the spectral classification of the flux standard Hiltner 102 based on new spectroscopic data.
• To assess the implications of a hotter companion star for the system's observed variability.

Proposed method

• Acquisition of far-red spectra of V635 Cas using ground-based telescopes.
• Spectral classification based on line profiles and equivalent widths of hydrogen lines, particularly Hα and Paschen series.
• Comparison of observed line profiles with models of decretion disks around Oe stars.
• Use of Hiltner 102 as a flux standard for calibration and spectral comparison.
• Application of standard spectral classification techniques to re-evaluate the spectral type of Hiltner 102.
• Analysis of temporal variability in line profiles between 1992 and other epochs to detect disk-loss events.

Experimental results

Research questions

• RQ1 What is the true optical spectral type of V635 Cassiopeiae/4U 0115+63, given its ambiguous earlier classification as a Be star?
• RQ2 What causes the extreme variability in Hα and Paschen lines observed in February 1992?
• RQ3 How do the line profiles in the far-red spectrum constrain the geometry and structure of the circumstellar disk?
• RQ4 Is the flux standard Hiltner 102 correctly classified as a B0 III star, or does new data suggest a different spectral type?
• RQ5 What role does the presence of a hotter companion star play in driving the observed variability?

Key findings

• V635 Cas is definitively classified as an O9e star, based on spectral features and line profiles.
• Extreme variability in Hα and Paschen lines from emission to absorption in February 1992 is interpreted as a disk-loss event, the first such observation in this system.
• The line profiles and equivalent widths are consistent with a late-type Oe star with a decretion disk.
• The flux standard Hiltner 102 is reclassified as an O9.7 II star with a slight nitrogen enhancement.
• The observed variability is attributed in part to the presence of a hotter companion star.
• The far-red spectra provide strong evidence for a circumstellar disk with variable ionization and density structure.