Infrared studies of the Be star X Per

Abstract

Photometric and spectroscopic results are presented for the Be star X Per/HD 24534 from nearinfrared (near-IR) monitoring in 2010-2011. The star is one of a sample of selected Be/X-ray binaries being monitored by us in the near-IR to study correlations between their X-ray and near-IR behaviour. Comparison of the star's present near-IR magnitudes with earlier records shows the star to be currently in a prominently bright state with mean J, H, K magnitudes of 5.49, 5.33 and 5.06, respectively. The JHK spectra are dominated by emission lines of He I and Paschen and Brackett lines of H I. Lines of O I 1.1287 and 1.3165 μm are also present and their relative strength indicates, since O I 1.1287 is the stronger among the two lines, that Lyman β fluorescence plays an important role in their excitation. Recombination analysis of the H I lines is done, which shows that the Paschen and Brackett line strengths deviate considerably from case B predictions. These deviations are attributed to the lines being optically thick, and this supposition is verified by calculating the line centre optical depths predicted by recombination theory. Similar calculations indicate that the Pfund and Humphrey series lines should also be expected to be optically thick, which is found to be consistent with observations reported in other studies. The spectral energy distribution of the star is constructed and shown to have an IR excess. Based on the magnitude of the IR excess, which is modelled using a free-free contribution from the disc, the electron density in the disc is estimated and shown to be within the range of values expected in Be star discs.

Optical spectroscopy of Galactic field classical Be stars

“…It was Hiltner (1947) who first identified Ca triplet emission in the spectra of CBe stars. CBe star discs are known to be hot in nature, with disc temperature ranging within 10,000 -20,000K (Mathew et al 2012a;Sigut & Jones 2007). Hence, Ca lines are not expected to be seen in emission for CBe stars.…”

“…They calculated that for non -shell CBe stars, the mean electron density in the discs of their program stars not to be exceeding 10 12 cm -3 . Mathew et al (2012a) estimated the electron density for the star X Per to be within 10 11 -10 13 cm -3 . This implies that CBe star envelopes are optically thick where the average electron density (n e ) is significantly larger than what is found in the nebular regions.…”

Studies of the Be star X Persei during a bright infrared phase

“…The orbital period of the system was inferred from RXT E data to be 250 days while the eccentricity is 0.11 and orbital inclination angle is in between 26 and 33 degrees (Delgado-Martí et al 2001). Mathew et al (2012b) did infrared photometric and spectroscopic studies of X Per during 2010 December 4 to 2011 February 8 and found the object to be in a photometrically bright state in the near-IR compared to earlier recorded values. Large optical depth effects were also observed in the Hi recombination lines as implied from the deviation of Paschen and Brackett line strengths from recombination Case B values.…”

Optical/Infrared properties of Be stars in X-ray Binary systems

“…Abu. The details of the monitoring observations and data analysis are presented in Mathew et al (2012;. During our NIR observations, the Be star was observed to be prominently bright in the JHK-bands with mean magnitudes of 5.49, 5.33 and 5.06, respectively.…”