We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg super(2) and span distances ranging from 3 to 30 kpc from ...the Galactic center. Object selection is based on phased, photometric data with 28-50 epochs. We use this large sample to explore the bulk properties of the stellar halo, including the spatial distribution. The period-amplitude distribution of this sample shows that the majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a significant fraction (26%) which have longer periods and appear to be Oosterhoff type II. We find that the radial distributions of these two populations have significantly different profiles (image and image). This suggests that the stellar halo was formed by at least two distinct accretion processes and supports dual-halo models.
We describe the first results from the Outer Limits Survey, an NOAO survey designed to detect, map, and characterize the extended structure of the Large and Small Magellanic Clouds (LMC and SMC). The ...survey consists of deep images of 55 06 X 06 fields distributed at distances up to 20? from the Clouds, with 10 fields at larger distances representing controls for contamination by Galactic foreground stars and background galaxies. The field locations probe the outer structure of both the LMC and SMC, as well as exploring areas defined by the Magellanic Stream, the Leading Arm, and the LMC orbit as recently measured from its proper motion. The images were taken with C, M, R, I, and DDO51 filters on the CTIO Blanco 4 m telescope and Mosaic2 camera, with supporting calibration observations taken at the CTIO 0.9 m telescope. The CRI images reach depths below the oldest main-sequence (MS) turnoffs at the distance of the Clouds, thus yielding numerous probes of structure combined with good ability to measure stellar ages and metallicities. The M and DDO51 images allow for discrimination of LMC and SMC giant stars from foreground dwarfs, allowing us to use giants as additional probes of Cloud structure and populations. From photometry of eight fields located at radii of 7?-19? north of the LMC bar, we find MS stars associated with the LMC out to 16? from the LMC center, while the much rarer giants can only be convincingly detected out to 11?. In one field, designated as a control, we see the unmistakable signature of the Milky Way (MW) globular cluster NGC 1851, which lies several tidal radii away from the field center. The color-magnitude diagrams show that while at 7? radius LMC populations as young as 500 Myr are present, at radii 11? only the LMC's underlying old metal-poor (M/H ~--1) population remains, demonstrating the existence of a mean population gradient at these radii. Nevertheless, even at extreme large distances, the dominant age is significantly younger than that of the Galactic globular clusters. The MS star counts follow an exponential decline with distance with a scale length of 1.15 kpc, essentially the same scale length as gleaned for the inner LMC disk from prior studies. While we cannot rule out the existence of undetected tidal features elsewhere in the LMC periphery, the detection of an ordered structure to 12 disk scale lengths is unprecedented and adds to the puzzle of the LMC's interaction history with the SMC and the MW. Our results do not rule out the possible existence of an LMC stellar halo, which we show may only begin to dominate over the disk at still larger radii than where we have detected LMC populations.
We present a measurement of the systemic proper motion of the Large Magellanic Cloud (LMC) from astrometry with the High Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) on the Hubble ...Space Telescope (HST). We observed LMC fields centered on 21 background QSOs that were discovered from their optical variability in the MACHO database. The QSOs are distributed homogeneously behind the central few degrees of the LMC. With two epochs of HRC data and a 62 yr baseline, we determine the proper motion of the LMC to better than 5% accuracy: k sub(W) = -2.03 c 0.08 mas yr super(-1), and k sub(N) = 0.44 c 0.05 mas yr super(-1). This is the most accurate proper-motion measurement for any Milky Way satellite thus far. When combined with H I data from the Magellanic Stream, this should provide new constraints on both the mass distribution of the Galactic halo and models of the Stream.
ABSTRACT
We present photometric light curves of the stellar occultation event of the star UCAC4 341-187633 on 2018 August 15 by the minor planet (134340) Pluto. Photometric observations were carried ...out using the 2.1-m telescope at the San Pedro Mártir Observatory and the 1.3-m telescopes at Sites 2 and 3 of the Trans-Neptunian Automated Occultation Survey (TAOS II) project, and using a portable 0.4-m telescope from Bahía Asunción, Baja California Sur, Mexico. Different filters were used with the 2.1-m telescope and the TAOS II telescopes, whilst observations with the portable system were performed with no filter. The resulting light curves from the San Pedro Mártir Observatory show clear structures, with at least two bright spikes observed on ingress and one more observed on egress as the star traverses the atmosphere of the dwarf planet. The light curve from the portable telescope (440 km away) measured a longer duration for the occultation event, because the shadow of Pluto was observed at a lower latitude. Normalized light curves were created for the 2.1-m telescope, the Site 3 telescope of the TAOS II and the portable telescope. These normalized light curves show a difference in amplitude.
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 d. In this paper we apply statistical tests, the chi-squared 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 d, 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 d. With our data, the results we obtained imply that the VRJH-band PL relations are non-linear around a period of 10 d, while the K-band PL relation is (marginally) consistent with a single-line regression. The choice of a period of 10 d, 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 data set with these long-period Cepheids. By examining our data at maximum light, we also suggest arguments as to 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 the MACHO data, using different extinction maps, suggests that this non-linearity is real.
The oldest and most metal-poor Milky Way stars form a kinematically hot halo, which motivates the two major formation scenarios for our galaxy: extended hierarchical accretion and rapid collapse. RR ...Lyrae stars are excellent tracers of old and metal-poor populations. We measured the kinematics of 43 RR Lyrae stars in the inner regions of the nearby Large Magellanic Cloud (LMC) galaxy. The velocity dispersion equals 53 ± 10 kilometers per second, which indicates that a kinematically hot metal-poor old halo also exists in the LMC. This result suggests that our galaxy and smaller late-type galaxies such as the LMC have similar early formation histories.
We use the 8 year light-curve database from the MAssive Compact Halo Objects project together with infrared colors and magnitudes from the Two Micron All Sky Survey to identify a sample of 22,000 ...long-period variables in the Large Magellanic Cloud (referred to hereafter as LMC LPVs). A period-luminosity diagram of these stars reveals six well-defined sequences, in substantial agreement with previous analyses of samples from the Optical Gravitational Lensing Experiment. In our analysis we identify analogues to Galactic LPVs in the LMC LPV sample. We find that carbon-dominated asymptotic giant branch (AGB) stars populate only two of the sequences, one of which includes the Mira variables. The high-luminosity end of the same two sequences are also the location of the only stars with J - Ks > 2, indicating that they are enshrouded in dust. The unknown mechanism that drives the variability of stars in the longest period produces different morphology in the period-luminosity diagram as compared with the shortest period sequences, which are thought to be caused by pulsation. In particular, the longest period sequence extends to lower luminosity red giant branch stars, and the luminosity function does not peak among the AGB stars. We point out several features that will constrain new models of the period-luminosity sequences.
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