Kenneth J. Mighell

M32: Is there an Ancient, Metal-Poor Population?

We observed two fields near M32 with the ACS/HRC on board the Hubble Space Telescope, located at distances of about 1.8’ and 5.4’ (hereafter Fl and F2, respectively) from the center of M32. To obtain a very detailed and deep color‐magnitude diagram (CMD) and to look for short period variability, we obtained time‐series imaging of each field in 32‐orbit‐long exposures using the F435W (B) and F555W (V) filters, spanning a temporal range of 2 days per filter. We focus on our detection of variability on RR Lyrae variable stars, which represents the only way to obtain information about the presence of a very old population (larger than 10 Gyr) in M32 from optical data. Here we present results obtained from the detection of 31 RR Lyrae in these fields: 17 in Fl and 14 in F2.

ORBITAL PHASE VARIATIONS OF THE ECCENTRIC GIANT PLANET HAT-P-2b

We present the first secondary eclipse and phase curve observations for the highly eccentric hot Jupiter HAT-P-2b in the 3.6, 4.5, 5.8, and 8.0 μm bands of the Spitzer Space Telescope. The 3.6 and 4.5 μm data sets span an entire orbital period of HAT-P-2b (P = 5.6334729 d), making them the longest continuous phase curve observations obtained to date and the first full-orbit observations of a planet with an eccentricity exceeding 0.2. We present an improved non-parametric method for removing the intrapixel sensitivity variations in Spitzer data at 3.6 and 4.5 μm that robustly maps position-dependent flux variations. We find that the peak in planetary flux occurs at 4.39 ± 0.28, 5.84 ± 0.39, and 4.68 ± 0.37 hr after periapse passage with corresponding maxima in the planet/star flux ratio of 0.1138% ± 0.0089%, 0.1162% ± 0.0080%, and 0.1888% ± 0.0072% in the 3.6, 4.5, and 8.0 μm bands, respectively. Our measured secondary eclipse depths of 0.0996% ± 0.0072%, 0.1031% ± 0.0061%, 0.071%^(+0.029%)_(-0.013%), and 0.1392% ± 0.0095% in the 3.6, 4.5, 5.8, and 8.0 μm bands, respectively, indicate that the planet cools significantly from its peak temperature before we measure the dayside flux during secondary eclipse. We compare our measured secondary eclipse depths to the predictions from a one-dimensional radiative transfer model, which suggests the possible presence of a transient day side inversion in HAT-P-2bs atmosphere near periapse. We also derive improved estimates for the system parameters, including its mass, radius, and orbital ephemeris. Our simultaneous fit to the transit, secondary eclipse, and radial velocity data allows us to determine the eccentricity (e = 0.50910 ± 0.00048) and argument of periapse (ω = 188°.09 ± 0°.39) of HAT-P-2bs orbit with a greater precision than has been achieved for any other eccentric extrasolar planet. We also find evidence for a long-term linear trend in the radial velocity data. This trend suggests the presence of another substellar companion in the HAT-P-2 system, which could have caused HAT-P-2b to migrate inward to its present-day orbit via the Kozai mechanism.

WFPC2 Observations of the Carina Dwarf Spheroidal Galaxy.

We present our analysis of Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (~V) and F814W (~I) of the Carina dwarf spheroidal galaxy. The resulting V vs (V-I) color-magnitude diagrams reach V ~ 27.1 mag. The reddening of Carina is estimated to be E(V-I) = 0.08 +- 0.02 mag. A new estimate of the distance modulus of Carina, (m-M)_0 = 19.87 +- 0.11 mag, has been derived primarily from existing photometry in the literature. The apparent distance moduli in V and I were determined to be (m-M)_V = 20.05 +- 0.11 mag and (m-M)_I = 19.98 +- 0.12 mag, respectively. These determinations assumed that Carina has a metallicity of [Fe/H] = -1.9 +- 0.2 dex. This space-based observation, when combined with previous ground-based observations, is consistent with (but does not necessarily prove) the following star formation scenario. The Carina dwarf spheroidal galaxy formed its old stellar population in a short burst (=< 3 Gyr) at about the same time the Milky Way formed its globular clusters. The dominant burst of intermediate-age star formation then began in the central region of the galaxy where stars formed for several billion years before the process of star formation became efficient enough in the outer regions of the galaxy to allow for the formation of large numbers of stars. There has been negligible star formation during the last few billion years. This observation provides evidence that at least some dwarf galaxies can have complex global star formation histories with local variations of the rate of star formation as a function of time and position within the galaxy.

The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32

We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometry based on Hubble Space Telescope ACS/HRC images for a region 110 00 from the center of M32 (F1) and a background field (F2) about 320 00 away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data (the color-magnitude diagram of M32 goes as deep as F435W 28:5 at 50% completeness level) we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images, as its photometric errors are 2 smaller than other methods tried. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe=H] = -0:2 dex in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch bump in M32 which, together with the red clump, allow us to constrain the age and metallicity of the dominant population in this region of M32. These features indicate that the mean age of M32’s population at 2 0 from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of asymptotic giant branch stars rising to MF555W -2:0. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or blue straggler stars in M32. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe=H] -0:2. There is not a noticeable presence of blue horizontal branch stars, suggesting that an ancient population with [Fe=H]< -1:3 does not significantly contribute to the light or mass of M32 in our observed fields. M32’s dominant population of 8‐10 Gyr implies a formation redshift of 1 . z f . 2, precisely when observations of the specific star formation rates and models of “downsizing” imply galaxies of M32’s mass ought to be forming their stars. Our CMD therefore provides a “ground-truth” of downsizing scenarios at z = 0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. Its CMD exhibits a broad color spread of red giant stars indicative of its metallicity range with a peak at [Fe=H] -0:4 dex, slightly more metal-poor than M32 in our fields. The observed blue plume consists of stars as young as 0.3 Gyr, in agreement with previous works on the disk of M31. The detection of bright AGB stars reveals the presence of intermediate-age population in M31, which is however less significant than that in M32 at our field’s location. Subject headings: Local Group — galaxies: individual: M32, M31 — galaxies: elliptical and lenticular, cD — galaxies: stellar content

RR LYRAE VARIABLES IN M32 AND THE DISK OF M31

We observed two fields near M32 with the Advanced Camera for Surveys/High Resolution Channel (ACS/HRC) on board the Hubble Space Telescope. The main field, F1, is 1farcm8 from the center of M32; the second field, F2, constrains the M31 background, and is 5farcm4 distant. Each field was observed for 16 orbits in each of the F435W (narrow B) and F555W (narrow V) filters. The duration of the observations allowed RR Lyrae stars to be detected. A population of RR Lyrae stars determined to belong to M32 would prove the existence of an ancient population in that galaxy, a subject of some debate. We detected 17 RR Lyrae variables in F1 and 14 in F2. A 1σ upper limit of 6 RR Lyrae variables belonging to M32 is inferred from these two fields alone. Use of our two ACS/WFC parallel fields provides better constraints on the M31 background, however, and implies that 7+4 -3 (68% confidence interval) RR Lyrae variables in F1 belong to M32. We have therefore found evidence for an ancient population in M32. It seems to be nearly indistinguishable from the ancient population of M31. The RR Lyrae stars in the F1 and F2 fields have indistinguishable mean V-band magnitudes, mean periods, distributions in the Bailey diagram, and ratios of RRc to RRtotal types. However, the color distributions in the two fields are different, with a population of red RRab variables in F1 not seen in F2. We suggest that these might be identified with the detected M32 RR Lyrae population, but the small number of stars rules out a definitive claim. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with GO proposal 10572.

THE STAR FORMATION HISTORY OF M32

We use deep Hubble Space Telescope Advanced Camera for Surveys/High Resolution Channel observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5 Gyr old stars contribute ~40% ± 17% of M32s mass, while ~55% ± 21% of M32s mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are = 6.8 ± 1.5 Gyr and = –0.01 ± 0.08 dex. The SFH additionally indicates the presence of young ( = 9.2 ± 1.2 Gyr and = –0.10 ± 0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31s disk at F2 (~1 disk scale length) to be ~5-9 Gyr, our results agree with an inside-out disk formation scenario for M31s disk.

The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32: Evidence for Intermediate-age Populations

We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometry based on Hubble Space Telescope ACS/HRC images for a region 110 00 from the center of M32 (F1) and a background field (F2) about 320 00 away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data (the color-magnitude diagram of M32 goes as deep as F435W 28:5 at 50% completeness level) we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images, as its photometric errors are 2 smaller than other methods tried. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe=H] = -0:2 dex in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch bump in M32 which, together with the red clump, allow us to constrain the age and metallicity of the dominant population in this region of M32. These features indicate that the mean age of M32’s population at 2 0 from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of asymptotic giant branch stars rising to MF555W -2:0. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or blue straggler stars in M32. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe=H] -0:2. There is not a noticeable presence of blue horizontal branch stars, suggesting that an ancient population with [Fe=H]< -1:3 does not significantly contribute to the light or mass of M32 in our observed fields. M32’s dominant population of 8‐10 Gyr implies a formation redshift of 1 . z f . 2, precisely when observations of the specific star formation rates and models of “downsizing” imply galaxies of M32’s mass ought to be forming their stars. Our CMD therefore provides a “ground-truth” of downsizing scenarios at z = 0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. Its CMD exhibits a broad color spread of red giant stars indicative of its metallicity range with a peak at [Fe=H] -0:4 dex, slightly more metal-poor than M32 in our fields. The observed blue plume consists of stars as young as 0.3 Gyr, in agreement with previous works on the disk of M31. The detection of bright AGB stars reveals the presence of intermediate-age population in M31, which is however less significant than that in M32 at our field’s location. Subject headings: Local Group — galaxies: individual: M32, M31 — galaxies: elliptical and lenticular, cD — galaxies: stellar content

RR LYRAE VARIABLES IN TWO FIELDS IN THE SPHEROID OF M31

We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32—between 4 and 6 kpc from the center of M31. The data cover a time baseline sufficient for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is = 25.29 ± 0.05, where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey diagram and the ratio of c-type RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are = 0.557 ± 0.003 and = 0.327 ± 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the ab-type RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at = –1.50 ± 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B – V) = 0.08 ± 0.03, we find a distance modulus of (m – M)_0 = 24.46 ± 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center.

A new member of the Local Group : the Tucana dwarf galaxy

Observations of the recently discovered dwarf galaxy in Tucana, which is a likely member of the Local Group, are presented. The system is highly flattened and classified as dE5. An upper limit on the distance modulus of 24.75 is determined from a V, V-I color-magnitude diagram. This galaxy has a B-R color of 1.1, and with the upper limit to the distance modulus, an absolute R magnitude of −10. This apparently isolated dwarf system, being on the opposite side of the Milky Way from most of the Local Group, may provide important constraints on Local Group dynamics

The Variable Sky of Deep Synoptic Surveys

The discovery of variable and transient sources is an essential product of synoptic surveys. The alert stream will require filtering for personalized criteria -- a process managed by a functionality commonly described as a Broker. In order to understand quantitatively the magnitude of the alert generation and Broker tasks, we have undertaken an analysis of the most numerous types of variable targets in the sky -- Galactic stars, QSOs, AGNs and asteroids. It is found that LSST will be capable of discovering ~10^5 high latitude |b| > 20 deg) variable stars per night at the beginning of the survey. (The corresponding number for |b| 10^5 per night, and if orbital determination has a 50% success rate per epoch, will drop below 1000/night within 2 years.