An a priori investigation of astrophysical false positives in ground-based transiting planet surveys

Astrophysical false positives due to stellar eclipsing binaries pose one of the greatest challenges to ground-based surveys for transiting Hot Jupiters. We have used known properties of multiple star systems and Hot Jupiter systems to predict, a priori, the number of such false detections and the number of genuine planet detections recovered in two hypothetical but realistic ground-based transit surveys targeting fields close to the galactic plane (b~10 degrees): a shallow survey covering a magnitude range 10<V<13, and a deep survey covering a magnitude range 15<V<19. Our results are consistent with the commonly-reported experience of false detections outnumbering planet detections by a factor of ~10 in shallow surveys, while in our synthetic deep survey we find ~1-2 false detections for every planet detection. We characterize the eclipsing binary configurations that are most likely to cause false detections and find that they can be divided into three main types: (i) two dwarfs undergoing grazing transits, (ii) two dwarfs undergoing low-latitude transits in which one component has a substantially smaller radius than the other, and (iii) two eclipsing dwarfs blended with one or more physically unassociated foreground stars. We also predict that a significant fraction of Hot Jupiter detections are blended with the light from other stars, showing that care must be taken to identify the presence of any unresolved neighbors in order to obtain accurate estimates of planetary radii. This issue is likely to extend to terrestrial planet candidates in the CoRoT and Kepler transit surveys, for which neighbors of much fainter relative brightness will be important.Comment: 33 pages, 7 figures, 4 tables; To be published in The Astrophysical Journa

Globular Cluster Formation in M82

We present high resolution mid-infrared (mid-IR; 11.7 and 17.65 micron) maps of the central 400 pc region of the starburst galaxy M82. Seven star forming clusters are identified which together provide ~ 15% of the total mid-IR luminosity of the galaxy. Combining the mid-IR data with thermal radio measurements and near- and mid-IR line emission, we find that these young stellar clusters have inferred masses and sizes comparable to globular clusters. At least 20% of the star formation in M82 is found to occur in super-star clusters.Comment: 12 pages including three color figures; accepted for publication in Ap

The Initial Mass Functions in the Super-Star-Clusters NGC 1569A and NGC 1705-1

I use recent photometric and stellar velocity dispersion measurements of the super-star-clusters (SSCs) NGC 1569A and NGC 1705-1 to determine their present-day luminosity/mass (L_V/M) ratios. I then use the inferred L_V/M ratios, together with population synthesis models of evolving star-clusters, to constrain the initial-mass-functions (IMFs) in these objects. I find that (L_V/M)_solar=28.9 in 1569A, and (L_V/M)_solar=126 in 1705-1. It follows that in 1569A the IMF is steep with alpha~2.5 for m**(-alpha)dm IMFs which extend to 0.1 M_sun. This implies that most of the stellar mass in 1569A is contained in low-mass (< 1 M_sun) stars. However, in 1705-1 the IMF is either flat, with alpha<2$, or it is truncated at a lower mass-limit between 1 and 3 M_sun. I compare the inferred IMFs with the mass functions (MFs) of Galactic globular clusters. It appears that 1569A has a sufficient reservoir of low-mass stars for it to plausibly evolve into an object similar to Galactic globular clusters. However, the apparent deficiency of low-mass stars in 1705-1 may make it difficult for this SSC to become a globular cluster. If low-mass stars do dominate the cluster mass in 1705-1, the large L_V/M ratio in this SSC may be evidence that the most massive stars have formed close to the cluster cores.Comment: ApJ, in press. 19 Pages, Latex; [email protected]

Hierarchical Structure Formation and Chemical Evolution of Galaxies

We present an analytical and phenomenological model for metal enrichment in halos based on hierarchical structure formation. This model assumes that astration of normal stellar populations along with SNe II already occurs at very high redshift. For halos that are not disrupted by SN II explosions, the chemical evolution of the gas and stars is explicitly determined by the rate of gas infall as compared with the astration rate and the corresponding rate of metal production by SNe II per H atom in the gas. This model provides a good description of the data on [Fe/H] for damped Ly alpha systems over a wide range of redshift 0.5 < z < 5. For all halos not disrupted by SN II explosions, if there is a cessation of gas infall, the metallicities of stars follow a bimodal distribution. This distribution is characterized by a sharp peak at the value of [Fe/H] corresponding to the time of infall cessation and by a broad peak at a higher value of [Fe/H] corresponding to the subsequent period of astration during which the bulk of the remaining gas forms stars. Such a distribution may be compared to that observed for the Galactic halo stars. If the gas in a halo is rapidly lost upon cessation of infall, then an assemblage of stars with a very sharply-defined [Fe/H] value will be left behind. This assemblage of stars may be accreted by a larger system and become a globular cluster of the larger system. We also discuss the masses and metallicities of the globular clusters in this model. (Abridged)Comment: 24 pages, 10 figures, to appear in Ap

A photometric investigation of the fields of two short-period Cepheids in Cygnus

xi, 95 leaves : ill. ; 28 cm.Includes abstract.Includes bibliographical references (leaves 90-93).Photoelectric, photographic and CCD photometry, as well as spectroscopic observations and proper motion data for stars in the fields of the short-period galactic Cepheids V1726 Cygni and SU Cygni are presented and analyzed. The existence of a sparsely populated cluster associated with V1726 Cyg has been confirmed, and a new, loose stellar group has been found in the vicinity of SU Cyg. The newly obtained distance modulus for C2128+488 (Anon. Platais), the cluster associated with V1726 Cyg, is V[subscript 0] - M[subscript V] = 10.98 [plus or minus] 0.02, corresponding to a distance of 1568 [plus or minus] 13 pc. The spatial coincidence and the close match of the radial velocity, proper motion and age of V1726 Cyg with those of the cluster indicate a high probability of cluster membership for the Cepheid. The space reddening of V1726 Cyg, found from two neighbouring stars, is E[subscript B-V] = 0.43 [plus or minus] 0.02 and its luminosity as a cluster member is = -3.42 [plus or minus] 0.07. The newly found group of stars in the vicinity of SU Cyg has a distance modulus of V[subscript 0] - M[subscript V] = 10.98 [plus or minus] 0.02 (d = 1040 pc). It contains mostly early A to late F-type stars, with a few early B-type stars whose membership is more uncertain. A reddening of E [subscript B-V] = 0.16 has been determined for SU Cyg from a nearby (20") star having an accurate MK spectral type. The probable discovery of a new planetary nebula, located about 3' south of SU Cyg, and a possible new cluster 17' west of SU Cyg, are also reported. (Abstract shortened by UMI.

Detection of a Temperature Inversion in the Broadband Infrared Emission Spectrum of TrES-4

We estimate the strength of the bandpass-integrated thermal emission from the extrasolar planet TrES-4 at 3.6, 4.5, 5.8, and 8.0 micron using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We find relative eclipse depths of 0.137 +/- 0.011%, 0.148 +/- 0.016%, 0.261 +/- 0.059%, and 0.318 +/- 0.044% in these four bandpasses, respectively. We also place a 2 sigma upper limit of 0.37% on the depth of the secondary eclipse in the 16 micron IRS peak-up array. These eclipse depths reveal that TrES-4 has an emission spectrum similar to that of HD 209458b, which requires the presence of water emission bands created by an thermal inversion layer high in the atmosphere in order to explain the observed features. TrES-4 receives more radiation from its star than HD 209458b and has a correspondingly higher effective temperature, therefore the presence of a temperature inversion in this planet's atmosphere lends support to the idea that inversions might be correlated with the irradiance received by the planet. We find no evidence for any offset in the timing of the secondary eclipse, and place a 3 sigma upper limit of |ecos(omega)|<0.0058 where e is the planet's orbital eccentricity and omega is the argument of pericenter. From this we conclude that tidal heating from ongoing orbital circulatization is unlikely to be the explanation for TrES-4's inflated radius.Comment: 10 pages in emulateapj format, 7 figures (some in color), accepted for publication in Ap

Absolute properties of the low-mass eclipsing binary CM Draconis

Spectroscopic and eclipsing binary systems offer the best means for determining accurate physical properties of stars, including their masses and radii. The data available for low-mass stars have yielded firm evidence that stellar structure models predict smaller radii and higher effective temperatures than observed, but the number of systems with detailed analyses is still small. In this paper we present a complete reanalysis of one of such eclipsing systems, CM Dra, composed of two dM4.5 stars. New and existing light curves as well as a radial velocity curve are modeled to measure the physical properties of both components. The masses and radii determined for the components of CM Dra are M1=0.2310+/-0.0009 Msun, M2=0.2141+/-0.0010 Msun, R1=0.2534+/-0.0019 Rsun, and R2=0.2396+/-0.0015 Rsun. With relative uncertainties well below the 1% level, these values constitute the most accurate properties to date for fully convective stars. This makes CM Dra a valuable benchmark for testing theoretical models. In comparing our measurements with theory, we confirm the discrepancies reported previously for other low-mass eclipsing binaries. These discrepancies seem likely to be due to the effects of magnetic activity. We find that the orbit of this system is slightly eccentric, and we have made use of eclipse timings spanning three decades to infer the apsidal motion and other related properties.Comment: 19 pages, 9 figures. Accepted for publication in Ap