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Register a new userStellar variability in open clusters. II. Discovery of a new period-luminosity relation in a class of fast-rotating pulsating stars in NGC 3766
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$Context.$ Pulsating stars are windows to the physics of stars enabling us to see glimpses of their interior. Not all stars pulsate, however. On the main sequence, pulsating stars form an almost continuous sequence in brightness, except for a magnitude range between $delta$ Scuti and slowly pulsating B stars. Against all expectations, 36 periodic variables were discovered in 2013 in this luminosity range in the open cluster NGC 3766, the origins of which was a mystery. $Aims.$ We investigate the properties of those new variability class candidates in relation to their stellar rotation rates and stellar multiplicity. $Methods.$ We took multi-epoch spectra over three consecutive nights using ESOs Very Large Telescope. $Results.$ We find that the majority of the new variability class candidates are fast-rotating pulsators that obey a new period-luminosity relation. We argue that the new relation discovered here has a different physical origin to the period-luminosity relations observed for Cepheids. $Conclusions.$ We anticipate that our discovery will boost the relatively new field of stellar pulsation in fast-rotating stars, will open new doors for asteroseismology, and will potentially offer a new tool to estimate stellar ages or cosmic distances.
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A recent photometric survey in the NGC~3766 cluster led to the detection of stars presenting an unexpected variability. They lie in a region of the Hertzsprung-Russell (HR) diagram where no pulsation are theoretically expected, in between the $delta$ Scuti and slowly pulsating B (SPB) star instability domains. Their variability periods, between $sim$0.1--0.7~d, are outside the expected domains of these well-known pulsators. The NCG~3766 cluster is known to host fast rotating stars. Rotation can significantly affect the pulsation properties of stars and alter their apparent luminosity through gravity darkening. Therefore we inspect if the new variable stars could correspond to fast rotating SPB stars. We carry out instability and visibility analysis of SPB pulsation modes within the frame of the traditional approximation. The effects of gravity darkening on typical SPB models are next studied. We find that at the red border of the SPB instability strip, prograde sectoral (PS) modes are preferentially excited, with periods shifted in the 0.2--0.5~d range due to the Coriolis effect. These modes are best seen when the star is seen equator-on. For such inclinations, low-mass SPB models can appear fainter due to gravity darkening and as if they were located between the $delta$~Scuti and SPB instability strips.
The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the instability strips of the delta Scuti and SPB stars. Moreover the periods of the new phenomenon, P~0.1-0.7 d, do not allow to associate it a priori to either of these two types of pulsations. Stars in the NGC 3766 cluster are known as fast rotators with rotational velocities typically larger than half of their critical velocity. Rotation can affect both the geometrical properties and period domain of pulsations. It also alters the apparent stellar luminosity through gravity darkening, effect seldom taken considered in theoretical studies of the rotation-pulsation interaction. We explore if both of these effects are able to deliver a consistent interpretation for the observed properties of the new variables in NGC 3766: explaining their presence outside the known instability strips and their variability periods. We carry out an instability analysis of SPB models within the framework of the Traditional Approximation of Rotation and study the visibility of modes according to the angle of view and rotation. We also check how gravity darkening affects the effective temperature and luminosity of stellar models for different angles of view and rotation velocities. At the red (cold) border of the instability strip, prograde sectoral modes are preferentially excited and their visibilities are maximum when seen equator-on. Furthermore low-mass SPB models seen equator-on can appear in the gap between non-rotating SPB and delta Scuti stars due to gravity darkening. In that case, periods of these most visible modes are shifted to the 0.2-0.5 d range due to the effects of the Coriolis force. We hence suggest that the new variable stars observed in NGC 3766 are actually fast rotating SPB pulsators.
Massive stars briefly pass through the yellow supergiant (YSG) phase as they evolve redward across the HR diagram and expand into red supergiants (RSGs). Higher-mass stars pass through the YSG phase again as they evolve blueward after experiencing significant RSG mass loss. These post-RSG objects offer us a tantalizing glimpse into which stars end their lives as RSGs, and why. One telltale sign of a post-RSG object may be an instability to pulsations, depending on the stars interior structure. Here we report the discovery of five YSGs with pulsation periods faster than 1 day, found in a sample of 76 cool supergiants observed by tess at two-minute cadence. These pulsating YSGs are concentrated in a HR diagram region not previously associated with pulsations; we conclude that this is a genuine new class of pulsating star, Fast Yellow Pulsating Supergiants (FYPS). For each FYPS, we extract frequencies via iterative prewhitening and conduct a time-frequency analysis. One FYPS has an extracted frequency that is split into a triplet, and the amplitude of that peak is modulated on the same timescale as the frequency spacing of the triplet; neither rotation nor binary effects are likely culprits. We discuss the evolutionary status of FYPS and conclude that they are candidate post-RSGs. All stars in our sample also show the same stochastic low-frequency variability (SLFV) found in hot OB stars and attributed to internal gravity waves. Finally, we find four $alpha$ Cygni variables in our sample, of which three are newly discovered.
$omega$ Centauri (NGC 5139) hosts hundreds of pulsating variable stars of different types, thus representing a treasure trove for studies of their corresponding period-luminosity (PL) relations. Our goal in this study is to obtain the PL relations for RR Lyrae, and SX Phoenicis stars in the field of the cluster, based on high-quality, well-sampled light curves in the near-infrared (IR). $omega$ Centauri was observed using VIRCAM mounted on VISTA. A total of 42 epochs in $J$ and 100 epochs in $K_{rm S}$ were obtained, spanning 352 days. Point-spread function photometry was performed using DoPhot and DAOPHOT in the outer and inner regions of the cluster, respectively. Based on the comprehensive catalogue of near-IR light curves thus secured, PL relations were obtained for the different types of pulsators in the cluster, both in the $J$ and $K_{rm S}$ bands. This includes the first PL relations in the near-IR for fundamental-mode SX Phoenicis stars. The near-IR magnitudes and periods of Type II Cepheids and RR Lyrae stars were used to derive an updated true distance modulus to the cluster, with a resulting value of $(m-M)_0 = 13.708 pm 0.035 pm 0.10$ mag, where the error bars correspond to the adopted statistical and systematic errors, respectively. Adding the errors in quadrature, this is equivalent to a heliocentric distance of $5.52pm 0.27$ kpc.
We investigate possible interpretations of the new periodic B- and A-type variable stars discovered in NGC 3766. They lie in the region of the Hertzsprung-Russell diagram between slowly pulsating B and delta Sct stars, a region where no pulsation is predicted by standard models of pulsating stars. We show that the two other possible causes of periodic light curve variations, rotational modulation and binarity, cannot provide a satisfactory explanation for all the properties observed in those stars either. The question of their origin is thus currently an open issue.
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