Variable Stars In NGC Research Articles

Variable Stars in the 50BiN Open Cluster Survey. III. NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way. Variable stars in open clusters provide a unique way to accurately probe the internal structure, temporal and dynamical evolutionary stages of individual stars and the host cluster. The most powerful tool for such studies is time-domain photometric observations. This paper follows the route of our previous work, concentrating on a photometric search for variable stars in NGC 884. The target cluster is the companion of NGC 869, forming the well-known double cluster system that is gravitationally bound. From the observation run in 2016 November, a total of 9247 B-band CCD images and 8218 V-band CCD images were obtained. We detected a total of 15 stars with variability in visual brightness, including five Be stars, three eclipsing binaries, and seven of unknown types. Two new variable stars were discovered in this work. We also compared the variable star content of NGC 884 with its companion NGC 869.

New Blue and Red Variable Stars in NGC 247

Abstract Images recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South are combined with archival images from other facilities to search for variable stars in the southern half of the nearby disk galaxy NGC 247. Fifteen new periodic and nonperiodic variables are identified. These include three Cepheids with periods <25 days, four semiregular variables, one of which shows light variations similar to those of R CrB stars, five variables with intrinsic visible/red brightnesses and colors that are similar to those of luminous blue variables (LBVs), and three fainter blue variables, one of which may be a noneclipsing close binary system. The S Doradus instability strip defines the upper envelope of a distinct sequence of objects on the (i, g−i) color–magnitude diagram (CMD) of NGC 247. The frequency of variability of an amplitude ≥0.1 magnitude in the part of the CMD that contains LBVs over the seven-month period when the GMOS images were recorded is ∼0.2. The light curve of the B[e] supergiant J004702.18–204739.9, which is among the brightest stars in NGC 247, is also examined. Low-amplitude variations on day-to-day timescales are found, coupled with a systematic trend in mean brightness over a six-month time interval.

New Variable Stars in NGC 6652 and Its Background Sagittarius Stream∗

Abstract We conducted a variable star search on the metal-rich Galactic globular cluster NGC 6652 using archival Gemini-S/Gemini Multiobject Spectrograph data. We report the discovery of nine new variable stars in the NGC 6652 field, of which we classify six as eclipsing binaries and one as an SX Phoenicis star, leaving two variables without classification. Using proper motions from Gaia DR2 and Hubble Space Telescope, albeit with some uncertainties, we find that the cluster, the field, and the background Sagittarius stream each have three of these variables. We also reassess the membership of known variables based on the Gaia proper motions, confirming the existence of one RR Lyrae star in the cluster.

Luminous and Variable Stars in NGC 2403 and M81*

Abstract We present the results of spectroscopy and multi-wavelength photometry of luminous and variable star candidates in the nearby spiral galaxies NGC 2403 and M81. We discuss specific classes of stars, the Luminous Blue Variables (LBVs), B[e] supergiants (sgB[e]), and the high-luminosity yellow hypergiants. We identify two new LBV candidates, and three sgB[e] stars in M81. We also find that some stars that were previously considered LBV candidates are actually field stars. The confirmed and candidate LBVs and sgB[e] stars together with the other confirmed members are shown on the HR Diagrams for their respective galaxies. We also present the HR Diagrams for the two “SN impostors”, V37 (SN2002kg) and V12(SN1954J) in NGC 2403 and the stars in their immediate environments.

TIME-SERIES PHOTOMETRY OF VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 288

We present the results of BV time-series photometry of the globular cluster NGC 288. Ob-servations were carried out to search for variable stars using the Korea Microlensing Telescope Network (KMTNet) 1.6-m telescopes and a 4k pre-science CCD camera during a test observation from August to December, 2014. We found a new SX Phe star and confirmed twelve previously known variable stars in NGC 288. For the semi-regular variable star V1, we newly determined a period of 37.3 days from light curves spanning 137 days. The light-curve solution of the eclipsing binary V10 indicates that the system is probably a detached system. The pulsation properties of nine SX Phe stars were examined by applying multiple frequency analysis to their light curves. We derived a new Period-Luminosity (P-L) relation, = −2.476(±0.300) logP −0.354(±0.385), from six SX Phe stars showing the fundamental mode. Additionally, the period ratios of three SX Phe stars that probably have a double-radial mode were investigated; P FO /P F = 0.779 for V5, P TO /P FO = 0.685 for V9, P SO /P FO = 0.811 for V11. This paper is the first contribution in a series assessing the detections and properties of variable stars in six southern globular clusters with the KMTNet system.

The puzzling new class of variable stars in NGC 3766: old friend pulsators?

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.

VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 2808

The first calibrated broadband BVI time-series photometry is presented for the variable stars in NGC 2808, with observations spanning a range of twenty-eight years. We have also redetermined the variability types and periods for the variable stars identified previously by Corwin et al, revising the number of probable fundamental-mode RR Lyrae variables (RR0) to 11 and the number of first-overtone variables (RR1) to five. Our observations were insufficient to discern the nature of the previously identified RR1 star, V24, and the tentatively identified RR1 star, V13. These two variables are \sim0.8 mag brighter than the RR Lyrae variables, appear to have somewhat erratic period and/or luminosity changes, and lie inside the RR Lyrae instability strip. Curiously, all but one of the RR Lyrae stars studied in this relatively metal-rich cluster exhibit the Blazhko phenomenon, an effect thought to occur with higher frequency in metal-poor environments. The mean periods of the RR0 and RR1 variables are <P>_RR0=0.56 pm 0.01 d and <P>_RR1=0.30 pm 0.02 d, respectively, supporting an Oosterhoff I classification of the cluster. On the other hand, the number ratio of RR1- to RR0-type variables is high, though not unprecedented, for an Oosterhoff I cluster. The RR Lyrae variables have no period shifts at a given amplitude as compared to the M3 variables, making it unlikely that these variables are He-enhanced. Using the recent recalibration of the RR Lyrae luminosity scale by Catelan & Cortes, a mean distance modulus of (m-M)_V= 15.57 pm 0.13 mag for NGC 2808 is obtained, in good agreement with that determined here from its type II Cepheid and SX Phoenicis population. Our data have also allowed the discovery of two new candidate SX Phoenicis stars and an eclipsing binary in the blue straggler region of the NGC 2808 color-magnitude diagram.

Periodic Variables and Gyrochronology in the Open Cluster NGC 2301

ABSTRACTWe present the results of a search for periodic variables within 4078 time-series light curves and an analysis of the period-color plane for stars in the field of the open cluster NGC 2301. One hundred thirty-eight periodic variables were discovered, of which five are eclipsing binary candidates with unequal minima. The remaining 133 periodic variables appear to consist mainly of late-type stars whose variation is due to rotation modulated by star spot activity. The determined periods range from less than a day to over 14 days and have nearly unreddened B - R colors in the range of 0.8 to 2.8. The Barnes () interpretation of the period-color plane of late type stars is tested with our data. Our data did not show distinct I and C sequences, likely due to nonmember field stars contaminating in the background, as we estimate the total contamination to be 43%. Using different assumptions, the gyrochronological age of the cluster is calculated to be 210 ± 25 Myr, which falls in the range of age values (164–250 Myr) determined by previous studies. Finally, we present evidence which nullifies the earlier suggestion that two of the variable stars in NGC 2301 might be white dwarfs.

The Araucaria project. Bright variable stars in NGC 6822 from a wide-field imaging survey

We have performed a search for variable stars in the dwarf irregular galaxy NGC 6822 using wide-field multi-epoch VI photometry down to a limiting magnitude $V$ $\sim$ 22. Apart from the Cepheid variables in this galaxy already reported in an earlier paper by Pietrzynski et al. (2004), we have found 1019 variable stars, 50 periodically variable stars with periods ranging from 0.12 to 66 days and 146 probably periodic variables. Twelve of these stars are eclipsing binaries and fifteen are likely new, low-amplitude Cepheids. Interestingly, seven of these Cepheid candidates have periods longer than 100 days, have very low amplitudes (less than 0.2 mag in $I$), and are very red. They could be young, massive Cepheids still embedded in dusty envelopes. The other objects span a huge range in colours and represent a mixture of different types of luminous variables. Many of the variables classified as non-periodic in the present study may turn out to be {\it periodic} variables once a much longer time baseline will be available to study them. We provide the catalogue of photometric parameters and show the atlas of light curves for the new variable stars. Our present catalogue is complementary to the one of Baldacci et al. (2005) which has focussed on very short-period and fainter variables in a subfield in NGC 6822.

The Araucaria Project. Variable stars outside the Local Group: NGC 300

ABSTRACT Wehave performed a search for variable stars in the Sculptor Group spiral galaxy NGC 300using wide-Þeld multi-epoch BV photometry up to a limiting magnitude V 23. Apart fromthe Cepheid variables in this galaxy already reported in an earlier paper by Pietrzynski« et al.and the variable blue supergiants reported by Bresolin et al., we have identiÞed 26 additionalperiodicvariables,withperiodsrangingfrom0.2to190d,and121slowvariables.Twoofthesenewly discovered variables are Galactic foreground stars (one of them is a W UMa system),and three correspond to previously identiÞed Wolf—Rayet stars. 16 stars are new Cepheids,and the other objects span a huge range in colours and represent a mixture of different typesof luminous variables. Key words: supergiants — Cepheids — stars: Wolf—Rayet — galaxies: stellar content. 1 INTRODUCTION Variablestarsingalaxiestracethehistoryoftheirstellarpopulations,and their study is important to understand the properties and currentevolutionary state of the host galaxy. In particular, the study of themost luminous variables in galaxies spanning a range of metallici-ties could enlighten the processes involved in the rather short, andsometimes spectacular, events observed near the end of the lives ofthe more massive stars.Past work on variable stars in galaxies has been mostly restrictedto the Local Group (Mateo 1998, and references therein). In con-trast, very few projects aimed at exploring the populations of vari-able stars outside the Local Group have been executed. One of thetargets which has received some attention over the last years, how-ever, mostly in the context of a revision of the extragalactic dis-tance scale in the framework of the Araucaria Project (Gieren et al.2001) has been the spiral galaxy NGC 300. NGC 300 is located inthe Sculptor Group at a distance of about 2 Mpc (Freedman et al.2001). Its metallicity is approximately bracketed by the Milky Wayand Small Magellanic Cloud (Urbaneja et al. 2003). Because ofits low inclination, NGC 300 is well suited to studies of its stel-lar content. Recent surveys include Wolf—Rayet stars (Schild et al.2003; Bresolin et al. 2002b), blue supergiants (Urbaneja et al. 2003;Bresolin et al. 2002a), Cepheids (Pietrzynski« et al. 2002), OB asso-ciations (Pietrzynski« et al. 2001) and supernova remnants (Pannutiet al. 2000). In this paper we use the photometric data base providedby Pietrzy«nski et al. (2002) to search for variable stars in NGC 300.In that paper, a search for Cepheid variables had already been con-ductedfromthesamedatasetandmorethanahundredofthemwerefound. This same observational material was very recently used by

The Cepheid Distance to NGC 1637: A Direct Test of the Expanding Photosphere Method Distance to SN 1999em

Type II-plateau supernovae (SNe II-P) are the classic variety of core-collapse events that result from isolated, massive stars with thick hydrogen envelopes intact at the time of explosion. Their distances are now routinely estimated through two techniques: the expanding photosphere method (EPM), a primary distance-determining method, and the recently developed standard-candle method (SCM), a promising secondary technique. Using Cycle 10 Hubble Space Telescope (HST) observations, we identify 41 Cepheid variable stars in NGC 1637, the host galaxy of the most thoroughly studied SN II-P to date, SN 1999em. Remarkably, the Cepheid distance that we derive to NGC 1637, D = 11.7 ± 1.0 Mpc, is nearly 50% larger than earlier EPM distance estimates to SN 1999em. This is the first direct comparison between these two primary distance-determining methods for a galaxy hosting a well-observed, spectroscopically and photometrically normal, SN II-P. Extensive consistency checks show strong evidence to support the Cepheid distance scale, so we are led to believe either that SN 1999em is in some heretofore unsuspected way an unusual SN II-P, or that the SN II-P distance scale must be revised. Assuming the latter, this one calibration yields H0(EPM) = 57 ± 15 km s-1 Mpc-1 and H0(SCM) = 59 ± 11 km s-1 Mpc-1; additional calibrating galaxies are clearly desirable in order to test the robustness of both determinations of H0. The HST observations of NGC 1637 also captured the fading SN 1999em 2 yr after explosion, providing the latest photometry ever obtained for an SN II-P. The nebular-phase photometric behavior of SN 1999em closely follows that observed for SN 1987A at similar epochs. The V and I light curves are both declining at rates significantly greater than the decay slope of 56Co predicts. This is likely due to an increasing transparency of the envelope to gamma rays and perhaps also to the formation of dust in the cooling atmosphere of the SN. The absolute V-band brightness of SN 1999em is ~0.25 mag brighter than SN 1987A at the same epochs, which suggests that a slightly greater amount of radioactive 56Ni, ~0.09 M☉, was ejected by SN 1999em than was derived for SN 1987A (0.075 M☉).

β Cephei and new pulsating stars in NGC 4755

We present the results of a photometric multi-colour, two-site observing campaign of β Cephei stars in the young open cluster NGC 4755. More than 260 h of time-series U, B, V filter data of the cluster were acquired on two continents during 1999 and 2000. We list the detected pulsation frequencies and discuss the results of their analysis. Mode identification of the detected modes is performed as well. In addition, we present the detection of four new variable stars in NGC 4755. We give frequency solutions as indicators of the time-scales and amplitudes of the pulsations. NGC 4755-116 is probably a B2 dwarf with a period of 4.2 d whose variability is caused by a spot or g-mode pulsation. NGC 4755-405 can be considered as a new β Cephei star with two pulsation frequencies. For NGC 4755-215 we found one frequency and for NGC 4755-316 three pulsation frequencies; we suggest that both are new slowly pulsating B stars of short period.

A search for new variable stars in NGC 6231

A search for new variable stars in NGC 6231

Variable stars in the globular cluster NGC 5634

view Abstract Citations (7) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Variable stars in the globular cluster NGC 5634 Liller, Martha H. ; Hogg, Helen Sawyer Abstract A photometric study with B plates of six of the seven known variable stars in NGC 5634 allows determination of the characteristics of the light variations. Three of the variables are of type RR the remaining three are RR . Two of the variables show evidence of period change. Publication: The Astronomical Journal Pub Date: August 1976 DOI: 10.1086/111931 Bibcode: 1976AJ.....81..628L full text sources ADS | data products SIMBAD (7)

Periods and Light-Curves of Variable Stars in NGC 121

82 photographs of the globular cluster NGC 121 on the outskirts of the Small Magellanic Cloud, covering a total range of 9 years but mostly concentrated in three consecutive seasons, are used to derive periods and light-curves of 5 variables within |$1^{\prime}$| of the cluster. Of these, three are short-period variables (periods between 0.50 and 0.64 days) of Bailey a and b type with the usual relations between period, amplitude and shape of light-curve. The other two, among the reddest and brightest of the cluster-stars, have periods of 140.2 and 112.4 : days. The light-variations are presented in the form of step-estimates on a “ basic ” sequence. Four other short-period variables have been found on the plates which are too far from the cluster to be members. They are sufficiently faint to be regarded as possibly the first instance of RR Lyr variables in the general field of the Small Magellanic Cloud.

The variable stars in the globular cluster Messier 28.

The variable stars in the globular cluster Messier 28.