Abstract:
Herbig Ae/Be stars are intermediate-mass pre-main sequence stars undergoing accretion through
their circumstellar disk. The optical and infrared (IR) spectra of HAeBe stars show Hi emission lines belonging
to Balmer, Paschen and Brackett series.We used the archival X-Shooter spectra available for 109 HAeBe stars
from Vioque et al. (2018) and analysed the various Hi lines present in them. We segregated the stars into
different classes based on the presence of higher-order lines in different Hi series.We discussed the dependence
of the appearance of higher-order lines on the stellar parameters.We found that most massive and younger stars
show all the higher-order lines in emission. The stars showing only lower-order lines have Teff < 12,000 K and
an age range of 5–10 Myr. We performed a case B line ratio analysis for a sub-sample of stars showing most
of the Hi lines in emission. We noted that all but four stars belonging to the sub-sample show lower Hi line
ratios than theoretical values, owing to the emitting medium being optically thick. The Hi line flux ratios do not
depend on the star’s spectral type. Further, from the line ratios of lower-order lines and Paschen higher-order
lines, we note that line ratios of most HAeBe stars match with electron density value in the range of 109–1011
cm−3. The electron temperature, however, could not be ascertained with confidence using the line ratios studied
in this work.
