Shashi M. Kanbur

Geometry of the Large Magellanic Cloud Disk: Results from MACHO and the Two Micron All Sky Survey

We present a detailed study of the viewing angles and geometry of the inner LMC (ρ 4°) based on a sample of more than 2000 MACHO Cepheids with complete {VR}KC light curves and single-phase Two Micron All Sky Survey (2MASS) JHKs observations. The sample is considerably larger than any previously studied subset of LMC Cepheids and has an improved areal coverage. Single-epoch random-phase 2MASS photometry is corrected using MACHO V light curves to derive mean JHKs magnitudes. We analyze the resulting period-luminosity relations in VRJHKs to recover statistical reddening and distance to each individual Cepheid, with respect to the mean distance modulus and reddening of the LMC. By fitting a plane solution to the derived individual distance moduli, the values of LMC viewing angles are obtained: position angle θ = 1510 ± 24, inclination i = 307 ± 11. In the so-called ring analysis, we find a strong dependence of the derived viewing angles on the adopted center of the LMC, which we interpret as being due to deviations from planar geometry. Analysis of residuals from the plane fit indicates the presence of a symmetric warp in the LMC disk and the bar elevated above the disk plane. Nonplanar geometry of the inner LMC explains a broad range for values of i and θ in the literature and suggests caution when deriving viewing angles from inner LMC data.

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?).

Period–colour and amplitude–colour relations in classical Cepheid variables

In this paper we analyse the behaviour of Galactic, Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) Cepheids in terms of period-colour (PC) and amplitude-colour (AC) diagrams at the phases of maximum, mean and minimum light. We find very different behaviour between Galactic and Magellanic Cloud Cepheids. Motivated by the recent report by Tammann et al., of a break in the LMC PC relations at 10 d, we use the statistical F-test to examine the PC relations at mean light in these three galaxies. The results of the F-test support the existence of the break occurring in the LMC PC(mean) relation, but not in the Galactic or SMC PC(mean) relation. Furthermore, the LMC Cepheids also show a break at minimum light, which is not seen in the Galactic and SMC Cepheids. We further discuss the effect on the period-luminosity relations in the LMC due to the break in the PC(mean) relation.

PERIOD-LUMINOSITY RELATIONS DERIVED FROM THE OGLE-III FUNDAMENTAL MODE CEPHEIDS

In this Paper, we have derived Cepheid period-luminosity (P-L) relations for the Large Magellanic Cloud (LMC) fundamental mode Cepheids, based on the data released from OGLE-III. We have applied an extinction map to correct for the extinction of these Cepheids. In addition to the V IW band P-L relations, we also include JHK and four Spitzer IRAC band P-L relations, derived by matching the OGLE-III Cepheids to the 2MASS and SAGE datasets, respectively. We also test the non-linearity of the Cepheid P-L relations based on extinction-corrected data. Our results (again) show that the LMC P-L relations are non-linear in V IJH bands and linear in KW and the four IRAC bands, respectively. Subject headings: Cepheids — distance scale

Further empirical evidence for the non‐linearity of the period–luminosity relations as seen in the Large Magellanic Cloud Cepheids

Recent studies, using OGLE data for LMC Cepheids in the optical, strongly suggest that the period-luminosity (PL) relation for the Large Magellanic Cloud (LMC) Cepheids shows a break or non-linearity at a period of 10 days. In this paper we apply statistical tests, the chi-square test and the F-test, to the Cepheid data from the MACHO project to test for a non-linearity of the V- and R-band PL relations at 10 days, and extend these tests to the near infrared (JHK-band) PL relations with 2MASS data. We correct the extinction for these data by applying an extinction map towards the LMC. The statistical test we use, the F-test, is able to take account of small numbers of data points and the nature of that data on either side of the period cut at 10 days. With our data, the results we obtained imply that the VRJH-band PL relations are non-linear around a period of 10 days, while the K-band PL relation is (marginally) consistent with a single-line regression. The choice of a period of 10 days, around which this non-linearity occurs, is consistent with the results obtained when this ”break” period is estimated from the data. We show that robust parametric (including least squares, least absolute deviation, robust regression) and non-parametric regression methods, which restrict the influence of outliers, produce similar results. Long period Cepheids are supplemented from the literature to increase our sample size. The photometry of these long period Cepheids is compared with our data and no trend with period is found. Our main results remain unchanged when we supplement our dataset with these long period Cepheids. By examining our data at maximum light, we also suggest arguments why errors in reddening are unlikely to be responsible for our results. The non-linearity of the mean V-band PL relation as seen in both of the OGLE and MACHO data, using different extinction maps, suggests that this non-linearity is real.

The extra-galactic Cepheid distance scale from LMC and Galactic period-luminosity relations

In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project and the Sandage-Tammann-Saha group. We use much of the Key Project methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean of a sparsely sampled Cepheid light curve, to published extra-galactic Cepheid data. We also apply different calibrating PL relations to estimate Cepheid distances, and investigate the sensitivity of the distance moduli to the adopted calibrating PL relation. We re-determine the OGLE LMC PL relations using a more conservative approach and also study the effect of using Galactic PL relations on the distance scale. For the Key Project galaxies after accounting for charge transfer effects, we find good agreement with an average discrepancy of -0.002 and 0.075 mag when using the LMC and Galaxy, respectively, as a calibrating PL relation. For NGC 4258 which has a geometric distance of 29.28 mag, we find a distance modulus of 29.44 ′ 0.06(random) mag, after correcting for metallicity. In addition we have calculated the Cepheid distance to 8 galaxies observed by the Sandage-Tammann-Saha group and find shorter distance moduli by -0.178 mag (mainly due to the use of different LMC PL relations) and -0.108 mag on average again when using the LMC and Galaxy, respectively, as a calibrating PL relation. However care must be taken to extrapolate these changed distances to changes in the resulting values of the Hubble constant because STS also use distances to NGC 3368 and 4414 and because STS calibration of SN Ia is often decoupled from the distance to the host galaxy through their use of differential extinction arguments. We also calculate the distance to all these galaxies using PL relations at maximum light and find very good agreement with mean light PL distances. However, after correcting for metallicity effects, the difference between the distance moduli obtained using the two sets of calibrating PL relations becomes negligible. This suggests that Cepheids in the LMC and Galaxy do follow different PL relations and constrains the sign for the coefficient of the metallicity correction, y, to be negative, at least at the median period log(P) 1.4, of the target galaxies.

The Period-Luminosity Relation for the Large Magellanic Cloud Cepheids Derived from Spitzer Archival Data

Using Spitzer archival data from the SAGE (Surveying the Agents of a Galaxys Evolution) program, we derive the Cepheid period-luminosity (P-L) relation at 3.6, 4.5, 5.8, and 8.0 μm for Large Magellanic Cloud (LMC) Cepheids. These P-L relations can be used, for example, in future extragalactic distance scale studies carried out with the James Webb Space Telescope. We also derive Cepheid period-color (P-C) relations in these bands and find that the slopes of the P-C relations are relatively flat. We test the nonlinearity of these P-L relations with the F-statistical test and find that the 3.6, 4.5, and 5.8 μm P-L relations are consistent with linearity. However, the 8.0 μm P-L relation presents possible but inconclusive evidence of nonlinearity.

The linearity of the Wesenheit function for the Large Magellanic Cloud Cepheids

There is strong evidence that the period-luminosity (PL) relation for the Large Magellanic Cloud (LMC) Cepheids shows a break at a period around 10 d. Because the LMC PL relation is extensively used in distance scale studies, the non-linearity of the LMC PL relation may affect the results based on this LMC calibrated relation. In this paper we show that this problem can be remedied by using the Wesenheit function in obtaining Cepheid distances. This is because the Wesenheit function is linear, although recent data suggest that the PL and the period-colour (PC) relations that make up the Wesenheit function are not. We test the linearity of the Wesenheit function and find strong evidence that the LMC Wesenheit function is indeed linear. This is because the non-linearity of the PL and PC relations cancel out when the Wesenheit function is constructed. We discuss this result in the context of distance scale applications. We also compare the distance moduli obtained from μ 0 = μ V - R(μ V - μ I ) (equivalent to Wesenheit functions) constructed with the linear and the broken LMC PL relations, and we find that the typical difference in distance moduli is ∼ +0.03 mag. Hence, the broken LMC PL relation does not seriously affect current distance scale applications. We also discuss the random error calculated with equation μ 0 = μ V - R(μ V - μ I ), and show that there is a correlation term that exists from the calculation of the random error. The calculated random error will be larger if this correlation term is ignored.

LARGE MAGELLANIC CLOUD NEAR-INFRARED SYNOPTIC SURVEY. I. CEPHEID VARIABLES AND THE CALIBRATION OF THE LEAVITT LAW

We present observational details and first results of a near-infrared (JHKs) synoptic survey of the central region of the Large Magellanic Cloud using the CPAPIR camera at the CTIO 1.5-m telescope. We covered 18 sq. deg. to a depth of Ks~16.5 mag and obtained an average of 16 epochs in each band at any given location. Our catalog contains more than 3.5x10^6 sources, including 1417 Cepheid variables previously studied at optical wavelengths by the OGLE survey. Our sample of fundamental-mode pulsators represents a 9-fold increase in the number of these variables with time-resolved, multi-band near-infrared photometry. We combine our large Cepheid sample and a recent precise determination of the distance to the LMC to derive a robust absolute calibration of the near-infrared Leavitt Law for fundamental-mode and first-overtone Cepheids with 10x better constraints on the slopes relative to previous work. We also obtain calibrations for the Tip of the Red Giant Branch and the Red Clump based on our ensemble photometry which are in good agreement with previous determinations.

Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling

We present the analysis of four first overtone RR Lyrae stars observed with the Kepler space telescope, based on data obtained over nearly 2.5yr. All four stars are found to be multiperiodic. The strongest secondary mode with frequency f2 has an amplitude of a few mmag, 20 45 times lower than the main radial mode with frequency f1. The two oscillations have a period ratio of P2/P1 = 0.612 0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is nonradial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P2/P1 �0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f2 at �1/2f2 and at �3/2f2. This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f2 and its subharmonics are variable on a timescale of 10 200d. The dominant radial mode also shows variations on the same timescale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1mmag, that must correspond to nonradial g-modes. Such modes never before have been observed in RR Lyrae variables.