* Astronomy

Blobrana · L

NGC 3766

Blobrana · L

Title: Stellar variability in open clusters. I. A new class of variable stars in NGC 3766
Authors: N. Mowlavi, F. Barblan, S. Saesen, L. Eyer

Aims. We analyse the population of periodic variable stars in the open cluster NGC 3766 based on a 7-year multi-band monitoring campaign conducted on the 1.2 m Swiss Euler telescope at La Silla, Chili.
Methods. The data reduction, light curve cleaning and period search procedures, combined with the long observation time line, allow us to detect variability amplitudes down to the milli-magnitude level. The variability properties are complemented with the positions in the colour-magnitude and colour-colour diagrams to classify periodic variable stars into distinct variability types.
Results. We find a large population (36 stars) of new variable stars between the red edge of slowly pulsating B (SPB) stars and the blue edge of delta Sct stars, a region in the Hertzsprung-Russell (HR) diagram where no pulsation is predicted to occur based on standard stellar models. The bulk of their periods ranges from 0.1 to 0.7 d, with amplitudes between 1 and 4 mmag for the majority of them. About 20% of stars in that region of the HR diagram are found to be variable, but the number of members of this new group is expected to be higher, with amplitudes below our milli-magnitude detection limit.
The properties of this new group of variable stars are summarised, and arguments set forth in favour of a pulsation origin of the variability, with g-modes sustained by stellar rotation. Potential members of this new class of low-amplitude periodic (most probably pulsating) A and late-B variables in the literature are discussed. We additionally identify 16 eclipsing binary, 13 SPB, 14 delta Sct and 12 gamma Dor candidates, as well as 72 fainter periodic variables. All are new discoveries.
Conclusions. We encourage to search for the existence of this new class of variables in other young open clusters, especially in those hosting a rich population of Be stars.

Read more (10965kb, PDF)

Blobrana · L

Title: The B and Be Star Population of NGC 3766
Authors: M. Virginia McSwain, Wenjin Huang, Douglas R. Gies, Erika D. Grundstrom, Richard H. D. Townsend

We present multiple epochs of H-alpha spectroscopy for 47 members of the open cluster NGC 3766 to investigate the long term variability of its Be stars. Sixteen of the stars in this sample are Be stars, including one new discovery. Of these, we observe an unprecedented 11 Be stars that undergo disk appearances and/or near disappearances in our H-alpha spectra, making this the most variable population of Be stars known to date. NGC 3766 is therefore an excellent location to study the formation mechanism of Be star disks. From blue optical spectra of 38 cluster members and existing Stromgren photometry of the cluster, we also measure rotational velocities, effective temperatures, and polar surface gravities to investigate the physical and evolutionary factors that may contribute to the Be phenomenon. Our analysis also provides improvements to the reddening and distance of NGC 3766, and we find E(B-V) = 0.22 ± 0.03 and (V-M_V)_0 = 11.6 ± 0.2, respectively. The Be stars are not associated with a particular stage of main-sequence evolution, but they are a population of rapidly rotating stars with a velocity distribution generally consistent with rotation at 70-80% of the critical velocity, although systematic effects probably underestimate the true rotational velocities so that the rotation is much closer to critical. Our measurements of the changing disk sizes are consistent with the idea that transitory, nonradial pulsations contribute to the formation of these highly variable disks.

Read more (981kb, PDF)