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At least 1,063 records · Page 59
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DOE OSTI2013

EVIDENCE FOR ENVIRONMENTAL DEPENDENCE OF THE UPPER STELLAR INITIAL MASS FUNCTION IN ORION A

We extend our previous study of the stellar population of L1641, the lower-density star-forming region of the Orion A cloud south of the dense Orion Nebula Cluster (ONC), with the goal of testing whether there is a statistically significant deficiency of high-mass stars in low-density regions. Previously, we compared the observed ratio of low-mass stars to high-mass stars with theoretical models of the stellar initial mass function (IMF) to infer a deficiency of the highest-mass stars in L1641. We expand our population study to identify the intermediate-mass (late B to G) L1641 members in an attempt to make a more direct comparison with the mass function of the nearby ONC. The spectral-type distribution and the K-band luminosity function of L1641 are similar to those of the ONC, but problems of incompleteness and contamination prevent us from making a detailed test for differences. We limit our analysis to statistical tests of the ratio of high-mass to low-mass stars, which indicate a probability of only 3% that the ONC and the southern region of L1641 were drawn from the same population, supporting the hypothesis that the upper-mass end of the IMF is dependent on environmental density.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DISK MASS-TO-LIGHT RATIO DISTRIBUTION FROM STELLAR POPULATION SYNTHESIS: APPLICATION TO ROTATION CURVE DECOMPOSITION OF NGC 5278 (KPG 390 A)

In this work we extend the study on the mass distribution of the spiral galaxy NGC 5278, performing 1D and 2D (GALFIT) bulge-disk decomposition to determine which components constitute the baryonic mass in this galaxy. Our analysis does not detect any bulge; instead we find a bright source probably related to the central active galactic nucleus and an exponential disk. We fix the stellar disk contribution to the rotation curve (RC) with broadband photometric observations and population synthesis models, to obtain the 2D mass distribution of the stellar disk. In the particular case of NGC 5278, we find that the typical assumption of considering the mass-to-luminosity ratio (M/L) of the disk as constant along the galactocentric radius is not valid. We also extract a baryonic RC from the mass profile to determine the inability of this baryonic RC and also the baryonic RC with more than and less than 30% disk mass (in order to consider the disk mass errors) to fit the entire RC. We perform the RC decomposition of NGC 5278 by considering the baryonic RC and four types of dark matter (DM) halo: Hernquist, Burkert, Navarro, Frenk, and White, and Einasto. Our results determine that the Hernquist DM halo better models our observed RC in the case of disk mass M{sub d} = 5.6 Multiplication-Sign 10{sup 10} M {sub Sun} and also with less than 30% disk mass. In the case of more than 30% disk mass, the cored Einasto (n < 4) DM halo is the best-fitting model.

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DOE OSTI2013

STELLAR POPULATIONS OF LYMAN BREAK GALAXIES AT z {approx_equal} 1-3 IN THE HST/WFC3 EARLY RELEASE SCIENCE OBSERVATIONS

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z {approx_equal} 1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 arcmin{sup 2} in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z {approx_equal} 1-3 are selected using dropout selection criteria similar to high-redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout-selected LBGs is accurate to within few percent; (2) the UV spectral slope {beta} is redder than at high redshift (z > 3), where LBGs are less dusty; (3) on average, LBGs at z {approx_equal} 1-3 are massive, dustier, and more highly star forming, compared to LBGs at higher redshifts with similar luminosities (0.1L* {approx}< L {approx}< 2.5L*), though their median values are similar within 1{sigma} uncertainties. This could imply that identical dropout selection technique, at all redshifts, finds physically similar galaxies; and (4) the stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of {approx}0.46, and star formation rates are proportional to their stellar masses with a logarithmic slope of {approx}0.90. These relations hold true-within luminosities probed in this study-for LBGs from z {approx_equal} 1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z {approx_equal} 2, but to avoid any selection biases, and for direct comparison with LBGs at z > 3, a true Lyman break selection at z {approx_equal} 2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties and their evolution.

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DOE OSTI2013

ENHANCED OFF-CENTER STELLAR TIDAL DISRUPTIONS BY SUPERMASSIVE BLACK HOLES IN MERGING GALAXIES

Off-center stellar tidal disruption flares have been suggested to be a powerful probe of recoiling supermassive black holes (SMBHs) out of galactic centers due to anisotropic gravitational wave radiations. However, off-center tidal flares can also be produced by SMBHs in merging galaxies. In this paper, we computed the tidal flare rates by dual SMBHs in two merging galaxies before the SMBHs become self-gravitationally bounded. We employ an analytical model to calculate the tidal loss-cone feeding rates for both SMBHs, taking into account two-body relaxation of stars, tidal perturbations by the companion galaxy, and chaotic stellar orbits in triaxial gravitational potential. We show that for typical SMBHs with masses 10{sup 7} M{sub Sun }, the loss-cone feeding rates are enhanced by mergers up to {Gamma} {approx} 10{sup -2} yr{sup -1}, about two orders of magnitude higher than those by single SMBHs in isolated galaxies and about four orders of magnitude higher than those by recoiling SMBHs. The enhancements are mainly due to tidal perturbations by the companion galaxy. We suggest that off-center tidal flares are overwhelmed by those from merging galaxies, making the identification of recoiling SMBHs challenging. Based on the calculated rates, we estimate the relative contributions of tidal flare events by single, binary, and dual SMBH systems during cosmic time. Our calculations show that the off-center tidal disruption flares by un-bound SMBHs in merging galaxies contribute a fraction comparable to that by single SMBHs in isolated galaxies. We conclude that off-center tidal disruptions are powerful tracers of the merging history of galaxies and SMBHs.

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DOE OSTI2013

THE BANANA PROJECT. IV. TWO ALIGNED STELLAR ROTATION AXES IN THE YOUNG ECCENTRIC BINARY SYSTEM EP CRUCIS: PRIMORDIAL ORIENTATION AND TIDAL ALIGNMENT

With observations of the EP Cru system, we continue our series of measurements of spin-orbit angles in eclipsing binary star systems, the BANANA project (Binaries Are Not Always Neatly Aligned). We find a close alignment between the sky projections of the rotational and orbital angular momentum vectors for both stars ({beta}{sub p} = -1. Degree-Sign 8 {+-} 1. Degree-Sign 6 and |{beta}{sub s}| < 17 Degree-Sign ). We also derive precise absolute dimensions and stellar ages for this system. The EP Cru and DI Her systems provide an interesting comparison: they have similar stellar types and orbital properties, but DI Her is younger and has major spin-orbit misalignments, raising the question of whether EP Cru also had a large misalignment at an earlier phase of evolution. We show that tidal dissipation is an unlikely explanation for the good alignment observed today, because realignment happens on the same timescale as spin-orbit synchronization, and the stars in EP Cru are far from synchronization (they are spinning nine times too quickly). Therefore it seems that some binaries form with aligned axes, while other superficially similar binaries are formed with misaligned axes.

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A WFC3/HST VIEW OF THE THREE STELLAR POPULATIONS IN THE GLOBULAR CLUSTER NGC 6752

Multi-band Hubble Space Telescope photometry reveals that the main sequence, sub-giant, and the red-giant branch of the globular cluster NGC 6752 splits into three main components in close analogy with the three distinct segments along its horizontal branch stars. These triple sequences are consistent with three stellar groups: a stellar population with a chemical composition similar to field-halo stars (Population a), a Population (c) with enhanced sodium and nitrogen, depleted carbon and oxygen, and an enhanced helium abundance ({Delta}Y {approx} 0.03), and a Population (b) with an intermediate (between Populations a and c) chemical composition and slightly enhanced helium ({Delta}Y {approx} 0.01). These components contain {approx}25% (Population a), {approx}45% (Population b), and {approx}30% (Population c) of the stars. No radial gradient for the relative numbers of the three populations has been identified out to about 2.5 half-mass radii.

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DOE OSTI2013

CENTRAL STELLAR MASS DEFICITS IN THE BULGES OF LOCAL LENTICULAR GALAXIES, AND THE CONNECTION WITH COMPACT z {approx} 1.5 GALAXIES

We have used the full radial extent of images from the Hubble Space Telescope's Advanced Camera for Surveys and Wide Field Planetary Camera 2 to extract surface brightness profiles from a sample of six, local lenticular galaxy candidates. We have modeled these profiles using a core-Sersic bulge plus an exponential disk model. Our fast rotating lenticular disk galaxies with bulge magnitudes M{sub V} {approx}< -21.30 mag have central stellar deficits, suggesting that these bulges may have formed from ''dry'' merger events involving supermassive black holes (BHs) while their surrounding disk was subsequently built up, perhaps via cold gas accretion scenarios. The central stellar mass deficits M{sub def} are roughly 0.5-2 M{sub BH} (BH mass), rather than {approx}10-20 M{sub BH} as claimed from some past studies, which is in accord with core-Sersic model mass deficit measurements in elliptical galaxies. Furthermore, these bulges have Sersic indices n {approx}3, half-light radii R{sub e} < 2 kpc and masses >10{sup 11} M{sub Sun }, and therefore appear to be descendants of the compact galaxies reported at z {approx} 1.5-2. Past studies which have searched for these local counterparts by using single-component galaxy models to provide the z {approx} 0 size comparisons have overlooked these dense, compact, and massive bulges in today's early-type disk galaxies. This evolutionary scenario not only accounts for what are today generally old bulges-which must be present in z {approx} 1.5 images-residing in what are generally young disks, but it eliminates the uncomfortable suggestion of a factor of three to five growth in size for the compact, z {approx} 1.5 galaxies that are known to possess infant disks.

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DOE OSTI2013

THE SUPERMASSIVE BLACK HOLE MASS-SPHEROID STELLAR MASS RELATION FOR SERSIC AND CORE-SERSIC GALAXIES

We have examined the relationship between supermassive black hole mass (M{sub BH}) and the stellar mass of the host spheroid (M{sub sph,*}) for a sample of 75 nearby galaxies. To derive the spheroid stellar masses we used improved Two Micron All Sky Survey K{sub s}-band photometry from the ARCHANGEL photometry pipeline. Dividing our sample into core-Sersic and Sersic galaxies, we find that they are described by very different M{sub BH}-M{sub sph,*} relations. For core-Sersic galaxies-which are typically massive and luminous, with M{sub BH} {approx}> 2 Multiplication-Sign 10{sup 8} M{sub Sun }-we find M{sub BH}{proportional_to} M{sub sph,*}{sup 0.97{+-}0.14}, consistent with other literature relations. However, for the Sersic galaxies-with typically lower masses, M{sub sph,*} {approx}< 3 Multiplication-Sign 10{sup 10} M{sub Sun }-we find M{sub BH}{proportional_to}M{sub sph,*}{sup 2.22{+-}0.58}, a dramatically steeper slope that differs by more than 2 standard deviations. This relation confirms that, for Sersic galaxies, M{sub BH} is not a constant fraction of M{sub sph,*}. Sersic galaxies can grow via the accretion of gas which fuels both star formation and the central black hole, as well as through merging. Their black hole grows significantly more rapidly than their host spheroid, prior to growth by dry merging events that produce core-Sersic galaxies, where the black hole and spheroid grow in lockstep. We have additionally compared our Sersic M{sub BH}-M{sub sph,*} relation with the corresponding relation for nuclear star clusters, confirming that the two classes of central massive object follow significantly different scaling relations.

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DOE OSTI2013

THE MAGELLANIC INTER-CLOUD PROJECT (MAGIC). I. EVIDENCE FOR INTERMEDIATE-AGE STELLAR POPULATIONS IN BETWEEN THE MAGELLANIC CLOUDS

The origin of the gas in between the Magellanic Clouds (MCs)-known as the ''Magellanic Bridge'' (MB)-is puzzling. Numerical simulations suggest that the MB formed from tidally stripped gas and stars in a recent interaction between the MCs. However, the apparent lack of stripped intermediate- or old-age stars associated with the MB is at odds with this picture. In this paper, we present the first results from the MAGellanic Inter-Cloud program (MAGIC) aimed at probing the stellar populations in the inter-Cloud region. We present observations of the stellar populations in two large fields located in between the Large and Small Magellanic Clouds (LMC/SMC), secured using the WFI camera on the 2.2 m telescope in La Silla. Using a synthetic color-magnitude diagram technique, we present the first quantitative evidence for the presence of intermediate-age and old stars in the inter-Cloud region. The intermediate-age stars-which make up {approx}28% of all stars in the region-are not present in fields at a similar distance from the SMC in a direction pointing away from the LMC. This provides potential evidence that these intermediate-age stars could have been tidally stripped from the SMC. However, spectroscopic studies will be needed to confirm or rule out the tidal origin for the inter-Cloud gas and stars.

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STELLAR CORONAE, SOLAR FLARES: A DETAILED COMPARISON OF {sigma} GEM, HR 1099, AND THE SUN IN HIGH-RESOLUTION X-RAYS

The Chandra High Energy Transmission Grating Spectrometer (HETG) spectra of the coronally active binary stars {sigma} Gem and HR 1099 are among the highest fluence observations for such systems taken at high spectral resolution in X-rays with this instrument. This allows us to compare their properties in detail to solar flare spectra obtained with the Russian CORONAS-F spacecraft's RESIK instrument at similar resolution in an overlapping bandpass. Here we emphasize the detailed comparisons of the 3.3-6.1 A region (including emission from highly ionized S, Si, Ar, and K) from solar flare spectra to the corresponding {sigma} Gem and HR 1099 spectra. We also model the larger wavelength range of the HETG, from 1.7 to 25 A - having emission lines from Fe, Ca, Ar, Si, Al, Mg, Ne, O, and N-to determine coronal temperatures and abundances. {sigma} Gem is a single-lined coronally active long-period binary which has a very hot corona. HR 1099 is a similar, but shorter period, double-lined system. With very deep HETG exposures we can even study emission from some of the weaker species, such as K, Na, and Al, which are important since they have the lowest first ionization potentials, a parameter well known to be correlated with elemental fractionation in the solar corona. The solar flare temperatures reach Almost-Equal-To 20 MK, comparable to the {sigma} Gem and HR 1099 coronae. During the Chandra exposures, {sigma} Gem was slowly decaying from a flare and its spectrum is well characterized by a collisional ionization equilibrium plasma with a broad temperature distribution ranging from 2 to 60 MK, peaking near 25 MK, but with substantial emission from 50 MK plasma. We have detected K XVIII and Na XI emission which allow us to set limits on their abundances. HR 1099 was also quite variable in X-rays, also in a flare state, but had no detectable K XVIII. These measurements provide new comparisons of solar and stellar coronal abundances, especially at the lowest first ionization potential (FIP) values. The low FIP elements do not show enhancement in the stellar coronae as they do in the Sun, except perhaps for K in {sigma} Gem. While {sigma} Gem and HR 1099 differ in their emission measure distributions, they have very similar elemental abundances.

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DOE OSTI2013

THE GALACTIC CENTER S-STARS AND THE HYPERVELOCITY STARS IN THE GALACTIC HALO: TWO FACES OF THE TIDAL BREAKUP OF STELLAR BINARIES BY THE CENTRAL MASSIVE BLACK HOLE?

In this paper, we investigate the link between the hypervelocity stars (HVSs) discovered in the Galactic halo and the Galactic center (GC) S-stars, under the hypothesis that they are both the products of the tidal breakup of the same population of stellar binaries by the central massive black hole (MBH). By adopting several hypothetical models for binaries to be injected into the vicinity of the MBH and doing numerical simulations, we realize the tidal breakup processes of the binaries and their follow-up dynamical evolution. We find that many statistical properties of the detected HVSs and GC S-stars could be reproduced under some binary injecting models, and their number ratio can be reproduced if the stellar initial mass function is top-heavy (e.g., with slope {approx} - 1.6). The total number of the captured companions is {approx}50 that have masses in the range {approx}3-7 M{sub Sun} and semimajor axes {approx}< 4000 AU and survive to the present within their main-sequence lifetime. The innermost one is expected to have a semimajor axis {approx}300-1500 AU and a pericenter distance {approx}10-200 AU, with a significant probability of being closer to the MBH than S2. Future detection of such a close star would offer an important test to general relativity. The majority of the surviving ejected companions of the GC S-stars are expected to be located at Galactocentric distances {approx}< 20 kpc, and have heliocentric radial velocities {approx} - 500-1500 km s{sup -1} and proper motions up to {approx}5-20 mas yr{sup -1}. Future detection of these HVSs may provide evidence for the tidal breakup formation mechanism of the GC S-stars.

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DOE OSTI2013

A SWIFT SURVEY OF ACCRETION ONTO STELLAR-MASS BLACK HOLES

We present a systemic analysis of all of the stellar-mass black hole binaries (confirmed and candidate) observed by the Swift observatory up to 2010 June. The broad Swift bandpass enables a trace of disk evolution over an unprecedented range in flux and temperature. The final data sample consists of 476 X-ray spectra containing greater than 100 counts, in the 0.6-10 keV band. This is the largest sample of high-quality CCD spectra of accreting black holes published to date. In addition, strictly simultaneous data at optical/UV wavelengths are available for 255 (54%) of these observations. The data are modeled with a combination of an accretion disk and a hard spectral component. For the hard component we consider both a simple power-law model and a thermal Comptonization model. An accretion disk is detected at greater than the 5{sigma} confidence level in 61% of the observations. Light curves and color-color diagrams are constructed for each system. Hardness-luminosity and disk fraction-luminosity diagrams are constructed and are observed to be consistent with those typically observed by RXTE, noting the sensitivity below 2 keV provided by Swift. The observed spectra have an average luminosity of {approx}1% Eddington, though we are sensitive to accretion disks down to a luminosity of 10{sup -3} L{sub Edd}. Thus, this is also the largest sample of such cool accretion disks studied to date. The accretion disk temperature distribution displays two peaks consistent with the classical hard and soft spectral states, with a smaller number of disks distributed between these. The distribution of inner disk radii is observed to be continuous regardless of which model is used to fit the hard continua. There is no evidence for large-scale truncation of the accretion disk in the hard state (at least for L{sub x} {approx}> 10{sup -3} L{sub Edd}), with all of the accretion disks having radii {approx}< 40 R{sub g} . Plots of the accretion disk inner radius versus hardness ratio reveal the disk radius to be decreasing as the spectrum hardens, i.e., enters the hard state. This is in contrast to expectations from the standard disk truncation paradigm and points toward a contribution from spectral hardening. The availability of simultaneous X-ray and optical/UV data for a subset of observations facilitates a critical examination of the role of disk irradiation via a modified disk model with a variable emissivity profile (i.e., T(r){proportional_to}r {sup -p}). The broadband spectra (X-ray-optical/UV) reveal irradiation of the accretion disk to be an important effect at all luminosities sampled herein, i.e., p {approx}< 0.75 for luminosities {approx}> 10{sup -3} L{sub Edd}. The accretion disk is found to dominate the UV emission irrespective of the assumed hard spectral component. Overall, we find the broadband soft-state spectra to be consistent with an irradiated accretion disk plus a corona, but we are unable to make conclusive statements regarding the nature of the hard-state accretion flow (e.g., ADAF/corona versus jet). Finally, the Swift data reveal a relation between the flux emitted by the accretion disk and that emitted by the corona, for this sample of stellar-mass black holes, that is found to be in broad agreement with the observed disk-corona relationship in Seyfert galaxies, suggesting a scale invariant coupling between the accretion disk and the corona.

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DOE OSTI2013

CHARACTERIZING THE STELLAR PHOTOSPHERES AND NEAR-INFRARED EXCESSES IN ACCRETING T TAURI SYSTEMS

Using NASA Infrared Telescope Facility SpeX data from 0.8 to 4.5 {mu}m, we determine self-consistently the stellar properties and excess emission above the photosphere for a sample of classical T Tauri stars (CTTS) in the Taurus molecular cloud with varying degrees of accretion. This process uses a combination of techniques from the recent literature as well as observations of weak-line T Tauri stars to account for the differences in surface gravity and chromospheric activity between the T Tauri stars and dwarfs, which are typically used as photospheric templates for CTTS. Our improved veiling and extinction estimates for our targets allow us to extract flux-calibrated spectra of the excess in the near-infrared. We find that we are able to produce an acceptable parametric fit to the near-infrared excesses using a combination of up to three blackbodies. In half of our sample, two blackbodies at temperatures of 8000 K and 1600 K suffice. These temperatures and the corresponding solid angles are consistent with emission from the accretion shock on the stellar surface and the inner dust sublimation rim of the disk, respectively. In contrast, the other half requires three blackbodies at 8000, 1800, and 800 K, to describe the excess. We interpret the combined two cooler blackbodies as the dust sublimation wall with either a contribution from the disk surface beyond the wall or curvature of the wall itself, neither of which should have single-temperature blackbody emission. In these fits, we find no evidence of a contribution from optically thick gas inside the inner dust rim.

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ISOTHERMAL DISTRIBUTIONS IN MONDian GRAVITY AS A SIMPLE UNIFYING EXPLANATION FOR THE UBIQUITOUS{rho}{proportional_to}r {sup -3} DENSITY PROFILES IN TENUOUS STELLAR HALOS

That the stellar halo of the Milky Way has a density profile which, to first approximation, satisfies{rho}{proportional_to}r {sup -3} and has been known for a long time. More recently, it has become clear that M31 also has such an extended stellar halo, which approximately follows the same radial scaling. Studies of distant galaxies have revealed the same phenomenology. Also, we now know that the density profiles of the globular cluster systems of our Galaxy and Andromeda to first approximation follow{rho}{proportional_to}r {sup -3},{Sigma}{proportional_to}R {sup -2} in projection. Recently, diffuse populations of stars have been detected spherically surrounding a number of Galactic globular clusters, extending much beyond the Newtonian tidal radii, often without showing any evidence of tidal features. Within the standard Newtonian and general relativity scenario, numerous and diverse particular explanations have been suggested, individually tailored to each of the different classes of systems described above. Here we show that in a MONDian gravity scenario any isothermal tenuous halo of tracer particles forming a small perturbation surrounding a spherically symmetric mass distribution will have an equilibrium configuration which to first approximation satisfies a{rho}{proportional_to}r {sup -3} scaling.

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DOE OSTI2013

THE STELLAR MASS GROWTH OF BRIGHTEST CLUSTER GALAXIES IN THE IRAC SHALLOW CLUSTER SURVEY

The details of the stellar mass assembly of brightest cluster galaxies (BCGs) remain an unresolved problem in galaxy formation. We have developed a novel approach that allows us to construct a sample of clusters that form an evolutionary sequence, and have applied it to the Spitzer IRAC Shallow Cluster Survey (ISCS) to examine the evolution of BCGs in progenitors of present-day clusters with mass of (2.5-4.5) Multiplication-Sign 10{sup 14} M{sub Sun }. We follow the cluster mass growth history extracted from a high resolution cosmological simulation, and then use an empirical method that infers the cluster mass based on the ranking of cluster luminosity to select high-z clusters of appropriate mass from ISCS to be progenitors of the given set of z = 0 clusters. We find that, between z = 1.5 and 0.5, the BCGs have grown in stellar mass by a factor of 2.3, which is well-matched by the predictions from a state-of-the-art semi-analytic model. Below z = 0.5 we see hints of differences in behavior between the model and observation.

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NUMERICAL SIMULATIONS OF THE POSSIBLE ORIGIN OF THE TWO SUB-PARSEC SCALE AND COUNTERROTATING STELLAR DISKS AROUND SgrA*

We present a high-resolution simulation of an idealized model to explain the origin of the two young, counterrotating, sub-parsec scale stellar disks around the supermassive black hole SgrA* at the center of the Milky Way. In our model, the collision of a single molecular cloud with a circumnuclear gas disk (similar to the one observed presently) leads to multiple streams of gas flowing toward the black hole and creating accretion disks with angular momentum depending on the ratio of cloud and circumnuclear disk material. The infalling gas creates two inclined, counterrotating sub-parsec scale accretion disks around the supermassive black hole with the first disk forming roughly 1 Myr earlier, allowing it to fragment into stars and get dispersed before the second counterrotating disk forms. Fragmentation of the second disk would lead to the two inclined, counterrotating stellar disks which are observed at the Galactic center. A similar event might be happening again right now at the Milky Way Galactic center. Our model predicts that the collision event generates spiral-like filaments of gas, feeding the Galactic center prior to disk formation with a geometry and inflow pattern that is in agreement with the structure of the so-called mini spiral that has been detected in the Galactic center.

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CALIBRATING STELLAR POPULATION MODELS WITH MAGELLANIC CLOUD STAR CLUSTERS

Stellar population models are commonly calculated using star clusters as calibrators for those evolutionary stages that depend on free parameters. However, discrepancies exist among different models, even if similar sets of calibration clusters are used. With the aim of understanding these discrepancies, and of improving the calibration procedure, we consider a set of 43 Magellanic Cloud (MC) clusters, taking age and photometric information from the literature. We carefully assign ages to each cluster based on up-to-date determinations, ensuring that these are as homogeneous as possible. To cope with statistical fluctuations, we stack the clusters in five age bins, deriving for each of them integrated luminosities and colors. We find that clusters become abruptly red in optical and optical-infrared colors as they age from {approx}0.6 to {approx}1 Gyr, which we interpret as due to the development of a well-populated thermally pulsing asymptotic giant branch (TP-AGB). We argue that other studies missed this detection because of coarser age binnings. Maraston and Girardi et al. models predict the presence of a populated TP-AGB at {approx}0.6 Gyr, with a correspondingly very red integrated color, at variance with the data; Bruzual and Charlot and Conroy models run within the error bars at all ages. The discrepancy between the synthetic colors of Maraston models and the average colors of MC clusters results from the now obsolete age scale adopted. Finally, our finding that the TP-AGB phase appears to develop between {approx}0.6 and 1 Gyr is dependent on the adopted age scale for the clusters and may have important implications for stellar evolution.

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THE STELLAR OBLIQUITY AND THE LONG-PERIOD PLANET IN THE HAT-P-17 EXOPLANETARY SYSTEM

We present the measured projected obliquity-the sky-projected angle between the stellar spin axis and orbital angular momentum-of the inner planet of the HAT-P-17 multi-planet system. We measure the sky-projected obliquity of the star to be {lambda}=19{sup +14}{sub -16} deg by modeling the Rossiter-McLaughlin effect in Keck/HIRES radial velocities (RVs). The anomalous RV time series shows an asymmetry relative to the midtransit time, ordinarily suggesting a nonzero obliquity-but in this case at least part of the asymmetry may be due to the convective blueshift, increasing the uncertainty in the determination of {lambda}. We employ the semi-analytical approach of Hirano et al. that includes the effects of macroturbulence, instrumental broadening, and convective blueshift to accurately model the anomaly in the net RV caused by the planet eclipsing part of the rotating star. Obliquity measurements are an important tool for testing theories of planet formation and migration. To date, the measured obliquities of {approx}50 Jovian planets span the full range, from prograde to retrograde, with planets orbiting cool stars preferentially showing alignment of stellar spins and planetary orbits. Our results are consistent with this pattern emerging from tidal interactions in the convective envelopes of cool stars and close-in planets. In addition, our 1.8 yr of new RVs for this system show that the orbit of the outer planet is more poorly constrained than previously thought, with an orbital period now in the range of 10-36 yr.

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