The present study makes use of the unprecedented capability of the Gaia mission to obtain the stellar parameters such as distance, age, and mass of HAeBe stars. The accuracy of Gaia DR2 astrometry is demonstrated from the comparison of the Gaia DR2 distances of 131 HAeBe stars with the previously estimated values from the literature. This is one of the initial studies to estimate the age and mass of a confirmed sample of HAeBe stars using both the photometry and distance from the Gaia mission. Mass accretion rates are calculated from Hα line flux measurements of 106 HAeBe stars. Since we used distances and the stellar masses derived from the Gaia DR2 data in the calculation of mass accretion rate, our estimates are more accurate than previous studies. The mass accretion rate is found to decay exponentially with age, from which we estimated a disk dissipation timescale of 1.9 ± 0.1 Myr. Mass accretion rate and stellar mass exhibits a power law relation of the form,Ṁ acc ∝ M 2.8±0.2 * . From the distinct distribution in the values of the infrared spectral index, n 2−4.6 , we suggest the possibility of difference in the disk structure between Herbig Be and Herbig Ae stars.

Narrow line Seyfert 1 galaxies (NLS1s) are believed to be powered by accretion of matter onto low mass black holes (BHs) in spiral host galaxies with BH masses M BH ∼ 10 6 -10 8 M . However, the broad band spectral energy distribution of the γ-ray emitting NLS1s are found to be similar to flat spectrum radio quasars. This challenges our current notion of NLS1s having low M BH . To resolve this tension of low M BH values in NLS1s, we fitted the observed optical spectrum of a sample of radio-loud NLS1s (RL-NLS1s), radio-quiet NLS1s (RQ-NLS1s) and radio-quiet broad line Seyfert 1 galaxies (RQ-BLS1s) of ∼500 each with the standard Shakura-Sunyaev accretion disk (AD) model. For RL-NLS1s we found a mean log(M AD BH /M ) of 7.98±0.54. For RQ-NLS1s and RQ-BLS1s we found mean log(M AD BH /M ) of 8.00±0.43 and 7.90±0.57, respectively. While the derived M AD BH values of RQ-BLS1s are similar to their virial masses, for NLS1s the derived M AD BH values are about an order of magnitude larger than their virial estimates. Our analysis thus indicates that NLS1s have M BH similar to RQ-BLS1s and their available virial M BH values are underestimated influenced by their observed relatively small emission line widths. Considering Eddington ratio as an estimation of the accretion rate and using M AD BH , we found the mean accretion rate of our RQ-NLS1s, RL-NLS1s and RQ-BLS1s as 0.06 +0.16 −0.05 , 0.05 +0.18 −0.04 and 0.05 +0.15 −0.04 , respectively. Our results therefore suggest that NLS1s have BH masses and accretion rates similar to BLS1s.

Most (∼82%) of the over 4000 confirmed exoplanets known today orbit very close to their host stars, within 0.5 au. Planets at such small orbital distances can result in significant interactions with their host stars, which can induce increased activity levels in them. In this work, we have searched for statistical evidence for Star-Planet Interactions (SPI) in the ultraviolet (UV) using the largest sample of 1355 GALEX detected host stars with confirmed exoplanets and making use of the improved host star parameters from Gaia DR2. From our analysis, we do not find any significant correlation between the UV activity of the host stars and their planetary properties. We further compared the UV properties of planet host stars to that of chromospherically active stars from the RAVE survey.Our results indicate that the enhancement in chromospheric activity of host stars due to star-planet interactions may not be significant enough to reflect in their near and far UV broad band flux.