Spectroscopic atlas of Halpha and Hbeta in a sample of northern Be stars

TL;DR

This study provides a detailed spectroscopic analysis of 48 northern Be stars, modeling their Halpha and Hbeta lines to estimate disk properties, variability, and stellar parameters, enhancing understanding of Be star circumstellar disks.

Contribution

It offers the first comprehensive modeling of Halpha emission in a sizable Be star sample, estimating disk radii, inclination angles, and variability patterns over nearly two years.

Findings

16 stars show variable equivalent width

7 stars exhibit profile variation over time

Maximum disk size ranges from 6 to 8 stellar radii

Abstract

We present Balmer lines spectroscopy for a sample of 48 Be stars. For most of them, H$\alpha$ and H$\beta$ have been observed more than two times, in a total period spanning almost two years between 2008 and 2009. Spectral synthesis of the H$\alpha$ profile was performed following two steps: photospheric contribution was computed by using Kurucz's code ATLAS9 and SYNTHE, and disk emission was derived by the approach of Hummel & Vrancken (2000, A&A, 302, 751). For 26 out of 48 stars, a modeling of the total H$\alpha$ emission has been attempted. By this modeling we derived an estimation of the disk radius, as well as the inclination angle between the rotational axis with line of sight and the base density at the stellar equator. For the stars observed more than once, we also discuss the variability of H$\alpha$ and H$\beta$ for what concerns both the equivalent width and the spectral profile. We found 16 stars with variable equivalent width and 7 stars with clear signs of profile variation. We derive effective temperature, gravity, and projected rotational velocity. We found 13 stars whose equivalent width is variable with a confidence level greater than 80% and 7 object for which spectral profiles show change with time. According to the commonly used classification scheme, we classified 16 stars as belonging to class 1, 13 to class 2, 11 are shell stars, 6 objects do not show net emission, and 2 stars display transitions from class 1 and 2. For the class 1 stars, we confirm the correlation between $v \sin i$ and peak separation. Concerning the geometry of the disk, we derived the base density at the stellar equator, the radius, and the inclination angle between rotational axis and line of sight. The maximum concentration of stars occurs for disk dimensions ranging in the interval of 6 to 8 stellar radii and for inclination angles going from 23$^\circ$ to $35^\circ$.