J. Harris

Paris Diderot University, Lutetia Parisorum, Île-de-France, France

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Publications (21)45.54 Total impact

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    ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two HII regions and 12 other objects, seven of which remain unclassified. (1 data file).
    VizieR Online Data Catalog. 09/2011; 741:11597.
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    ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H ii regions and 12 other objects, seven of which remain unclassified.
    Monthly Notices of the Royal Astronomical Society 02/2011; 411(3):1597 - 1627. · 5.52 Impact Factor
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    ABSTRACT: The Spitzer SAGE-Spec program (PID: 40159) consists of 224.6hr of spectroscopic observations of targets in the LMC. The targets included point sources and extended regions, both of which were observed using the IRS low-resolution and MIPS SED modes. Observations were done in the IRS staring mode for 196 point sources, and 48 point sources were observed in MIPS SED mode. In addition, 10 extended regions were mapped in both the MIPS SED and IRS observing modes. (4 data files).
    VizieR Online Data Catalog. 11/2010; 612:20683.
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    ABSTRACT: The SAGE-Spec Spitzer Legacy program is a spectroscopic follow-up to the SAGE-LMC photometric survey of the Large Magellanic Cloud carried out with the Spitzer Space Telescope. We present an overview of SAGE-Spec and some of its first results. The SAGE-Spec program aims to study the life cycle of gas and dust in the Large Magellanic Cloud, and to provide information essential to the classification of the point sources observed in the earlier SAGE-LMC photometric survey. We acquired 224.6 hours of observations using the InfraRed Spectrograph and the SED mode of the Multiband Imaging Photometer for Spitzer. The SAGE-Spec data, along with archival Spitzer spectroscopy of objects in the Large Magellanic Cloud, are reduced and delivered to the community. We discuss the observing strategy, the specific data reduction pipelines applied and the dissemination of data products to the scientific community. Initial science results include the first detection of an extragalactic "21 um" feature towards an evolved star and elucidation of the nature of disks around RV Tauri stars in the Large Magellanic Cloud. Towards some young stars, ice features are observed in absorption. We also serendipitously observed a background quasar, at a redshift of z~0.14, which appears to be host-less. Comment: 33 pages, 12 figures, 8 tables; accepted for publication by PASP
    04/2010;
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    ABSTRACT: Early results from the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the tidally-disrupted, low-metallicity Small Magellanic Cloud) Spitzer legacy program are presented. These early results concentrate on the SAGE-SMC MIPS observations of the SMC Tail region. This region is the high H i column density portion of the Magellanic Bridge adjacent to the SMC Wing. We detect infrared dust emission and measure the gas-to-dust ratio in the SMC Tail and find it similar to that of the SMC Body. In addition, we find two embedded cluster regions that are resolved into multiple sources at all MIPS wavelengths.
    IAU Symposium; 03/2009
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    ABSTRACT: We present deep, wide-field g and r photometry of the transition-type dwarf galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The data confirm the presence of both very young (<1 Gyr) and much older (>5 Gyr) stars. We study the structural properties of the old and young stellar populations by preferentially selecting either population on the basis of their color and magnitude. The young population is significantly more concentrated than the old population, with half-light radii of 104 ± 8 and 148 ± 16 pc, respectively, and their centers are slightly offset. Approximately 10% of the total stellar mass is estimated to be represented by the young stellar population. Comparison of the color-magnitude diagram (CMD) with theoretical isochrones, as well as numerical CMD fitting, suggests that star formation began over 10 Gyr ago and continued in recent times until at least a few hundred Myr ago. The CMD-fitting results are indicative of two distinct star formation bursts, with a quiescent period around 3 Gyr ago, albeit at low significance. The results are consistent with no metallicity evolution and a value of [ Fe/H ] ~ − 1.5 over the entire age of the system. Finally, the data show little, if any, sign of tidal distortion of Leo T.
    The Astrophysical Journal 12/2008; 680(2):1112. · 6.28 Impact Factor
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    ABSTRACT: We present MMT/Megacam imaging in Sloan g and r of the extremely low luminosity Boötes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and to calculate uncertainties on, Boötes II's distance, luminosity, size, and morphology. Comparisons with theoretical isochrones and empirical globular cluster fiducials show that Boötes II's stellar population is old and metal-poor ([Fe/H] − 2). Assuming a stellar population like that of M92, Boötes II is at a distance of 42 ± 2 kpc, closer than the initial published estimate of 60 ± 10 kpc. This distance revision, combined with a more robust measurement of Boötes II's structure with both a Plummer model and an exponential model, results in more compact inferred physical half-light sizes of rh 36 ± 9 and 33 ± 10 pc, respectively, and lower-limit inferred luminosities of MV − 2.4 ± 0.7 and -2.2 ± 0.7 mag, respectively. Our revised size and luminosity calculations move Boötes II into a region of size-luminosity space not previously known to be occupied by old stellar populations, but also occupied by the recently discovered Milky Way satellites Willman 1 and Segue 1. We show that the apparently distorted morphology of Boötes II is not statistically significant given the present data. We use a tidal argument to support a scenario in which Boötes II is a dwarf galaxy (dark matter-dominated) rather than a globular cluster (not dark matter-dominated), although our inferred uncertainty on the M/L for Boötes II is substantial. Moreover, we cannot rule out the possibility that Boötes II is a star cluster on the verge of disruption like Palomar 5.
    The Astrophysical Journal 12/2008; 688(1):245. · 6.28 Impact Factor
  • Galaxies in the Local Volume, Edited by Koribalski, B.~S. and Jerjen, H, 01/2008: pages 289;
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    ABSTRACT: We present ~1000 new candidate Young Stellar Objects (YSOs) in the Large Magellanic Cloud selected from Spitzer Space Telescope data, as part of the Surveying the Agents of a Galaxy's Evolution (SAGE) Legacy program. The YSOs, detected by their excess infrared (IR) emission, represent early stages of evolution, still surrounded by disks and/or infalling envelopes. Previously, fewer than 20 such YSOs were known. The candidate YSOs were selected from the SAGE Point Source Catalog from regions of color-magnitude space least confused with other IR-bright populations. The YSOs are biased toward intermediate- to high-mass and young evolutionary stages, because these overlap less with galaxies and evolved stars in color-magnitude space. The YSOs are highly correlated spatially with atomic and molecular gas, and are preferentially located in the shells and bubbles created by massive stars inside. They are more clustered than generic point sources, as expected if star formation occurs in filamentary clouds or shells. We applied a more stringent color-magnitude selection to produce a subset of "high-probability" YSO candidates. We fitted the spectral-energy distributions (SEDs) of this subset and derived physical properties for those that were well fitted. The total mass of these well-fitted YSOs is ~2900 M_☉ and the total luminosity is ~2.1 × 10^6 L_☉ . By extrapolating the mass function with a standard initial mass function and integrating, we calculate a current star-formation rate of ~0.06 M_☉ yr^(–1), which is at the low end of estimates based on total ultraviolet and IR flux from the galaxy (~0.05 – 0.25 M_☉ yr^(–1)), consistent with the expectation that our current YSO list is incomplete. Follow-up spectroscopy and further data mining will better separate the different IR-bright populations and likely increase the estimated number of YSOs. The full YSO list is available as electronic tables, and the SEDs are available as an electronic figure for further use by the scientific community.
    The Astronomical Journal 01/2008; · 4.97 Impact Factor
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    ABSTRACT: Color-magnitude diagrams (CMDs) are presented for the Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the Large Magellanic Cloud (LMC). IRAC and MIPS 24 um epoch one data are presented. These data represent the deepest, widest mid-infrared CMDs of their kind ever produced in the LMC. Combined with the 2MASS survey, the diagrams are used to delineate the evolved stellar populations in the Large Magellanic Cloud as well as Galactic foreground and extragalactic background populations. Some 32000 evolved stars brighter than the tip of the red giant branch are identified. Of these, approximately 17500 are classified as oxygen-rich, 7000 carbon-rich, and another 1200 as ``extreme'' asymptotic giant branch (AGB) stars. Brighter members of the latter group have been called ``obscured'' AGB stars in the literature owing to their dusty circumstellar envelopes. A large number (1200) of luminous oxygen--rich AGB stars/M supergiants are also identified. Finally, there is strong evidence from the 24 um MIPS channel that previously unexplored, lower luminosity oxygen-rich AGB stars contribute significantly to the mass loss budget of the LMC (1200 such sources are identified).
    The Astronomical Journal 09/2006; · 4.97 Impact Factor
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    Jason Harris, Dennis Zaritsky
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    ABSTRACT: We present the spatially-resolved star formation and chemical enrichment history of the Small Magellanic Cloud (SMC) across the entire central $4-degree x 4.5-degree area of the main body, based on UBVI photometry from our Magellanic Clouds Photometric Survey. We find that 1) approximately 50% of the stars that ever formed in the SMC formed prior to 8.4 Gyr ago (z >1.2 for WMAP cosmology), 2) the SMC formed relatively few stars between 8.4 and 3 Gyr ago, 3) there was a rise in the mean star formation rate during the most recent 3 Gyr punctuated by "bursts" at ages of 2.5, 0.4, and 0.06 Gyr, 4) the bursts at 2.5 and 0.4 Gyr are temporally coincident with past perigalactic passages of the SMC with the Milky Way, 5) there is preliminary evidence for a large-scale annular structure in the 2.5 Gyr burst, and 6) the chemical enrichment history derived from our analysis is in agreement with the age-metallicity relation of the SMC's star clusters. Consistent interpretation of the data required an ad hoc correction of 0.1-0.2 mag to the B-V colors of 25% of the stars; the cause of this anomaly is unknown, but we show that it does not strongly influence our results.
    The Astronomical Journal 01/2004; · 4.97 Impact Factor
  • J. Harris, D. Zaritsky
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    ABSTRACT: Based on the UBVI photometry of 6 Million stars in the Small Magellanic Cloud (SMC) from our Magellanic Clouds Photometric Survey, we present independent star-formation histories (SFHs) for 420 SMC subregions, covering the entire central 4x4 degrees of this galaxy. The collection of SFHs allows us to construct a detailed and comprehensive map of the history of star formation in the SMC. This work represents the most complete reconstruction of the SMC's history yet performed. We compare our results to a simple model in which star formation bursts are triggered by past close interactions with the LMC and Milky Way.
    12/2002;
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    ABSTRACT: We compare the distribution of stars of different spectral types, and hence mean age, within the central SMC and find that the asymmetric structures are almost exclusively composed of young main-sequence stars. Because of the relative lack of older stars in these features and the extremely regular distribution of red giant and clump stars in the SMC central body, we conclude that tides alone are not responsible for the irregular appearance of the central SMC. The dominant physical mechanism in determining the current-day appearance of the SMC must be star formation triggered by a hydrodynamic interaction between gaseous components. These results extend the results of population studies (see Gardiner & Hatzidimitriou) inward in radius and also confirm the suggestion of the spheroidal nature of the central SMC based on kinematic arguments (Dopita et al.; Hardy, Suntzeff, & Azzopardi). Finally, we find no evidence in the underlying older stellar population for a "bar" or "outer arm," again supporting our classification of the central SMC as a spheroidal body with highly irregular recent star formation.
    The Astrophysical Journal 05/2000; 534(1):L53-L56. · 6.28 Impact Factor
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    ABSTRACT: We report on our ongoing effort to reconstruct the spatially resolved star formation history (SFH) of the Magellanic Clouds. Our photometric survey of the Magellanic Clouds will eventually provide us with UBVI photometry and astrometry of tens of millions of stars in each galaxy, down to approximately V=21 mag. We are developing a new method to determine the SFH based on an iterative maximum likelihood comparison of the distribution of stars in photometric space to an analogous occupation probability map based on an input star formation history and a complete set of isochrones. We will resolve spatial variations in the SFH by dividing the data into spatial cells, and determining the SFH independently for each cell.
    Proceedings of the International Astronomical Union 01/1999; 190.
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    ABSTRACT: In contrast to the Milky Way the Magellanic Clouds contain a wealth of compact star clusters of all ages. In the LMC, approximately 1800 clusters were detected through photographic surveys, and the total number has been estimated to be ~ 4600. Color-magnitude diagrams (CMDs) are available only for a small subset so far. The Magellanic Cloud Photometric Survey (PI: Zaritsky) provides a unique and homogeneous database of UBVI CCD photometry of the Magellanic Clouds. The survey will double (possibly triple) the number of known star clusters in the Magellanic Clouds. Our new detections are mostly small, poor, or loose clusters. The survey helps to complete the cluster census toward the faint end of the cluster luminosity function and provides a census of star clusters as a function of brightness, color, age, and spatial distribution. The cluster CMDs allow us to determine isochrone-based ages for all detected clusters. For the majority of clusters, we are determining ages for the first time. For clusters with earlier age estimates from integrated colors we often find significant differences when using CMD-based ages. The positions of star clusters as a function of age allow us to trace the star formation history of the Magellanic Clouds. We are using the resulting age distribution function to explore the presence of gaps and peaks in cluster formation in both Clouds. Ages in correlation with structural parameters such as tidal radii and richness allow us to study cluster evolution and disruption as well as the evolution of the cluster luminosity function as a function of time. In addition, we use our isochrone-based ages for an observational age calibration of integrated UBVI colors, which will prove valuable also for interpreting the integrated colors of unresolved star clusters in distant galaxies and as input for population synthesis models.
    Proceedings of the International Astronomical Union 01/1999; 190:405.
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    ABSTRACT: We are undertaking a UBVI photometric survey of the central 8 by 8 degrees of the LMC and 4 by 4 degrees of the SMC. Although we are several years away from completing the survey, we have focused on obtaining results as the survey progresses. In this presentation, I will describe the basic parameters of the survey, the use of the Great Circle Camera, and some recent results. In particular, I will discuss our analysis of the data with regards to the distribution of dust in the LMC, the correlation function of stars and implications for dynamical evolution of stellar populations, and the recent (< few Gyr) star formation history of the LMC.
    Proceedings of the International Astronomical Union 01/1999; 190.
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    Proceedings of the International Astronomical Union 12/1998; 192:72.
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    ABSTRACT: Following an approach developed by Paczy\'nski & Stanek, we derive a distance to the Large Magellanic Cloud (LMC) by comparing red clump stars from the Hipparcos catalog with the red clump stars observed in two fields in the LMC that were selected from the ongoing photometric survey of the Magellanic Clouds to lie in low extinction regions. The use of red clump stars allows a single step determination of the distance modulus to the LMC, $\mu_{0,LMC} = 18.065\pm 0.031\pm 0.09 $mag (statistical plus systematic error), and the corresponding distance, $R_{LMC}= 41.02\pm 0.59\pm 1.74 kpc$. This measurement is in excellent agreement with the recent determination by Udalski et al., also based on the red clump stars, but is $\sim 0.4 $mag smaller than the generally accepted value of $\mu_{0,LMC} = 18.50\pm 0.15 $mag. We discuss possible reasons for this discrepancy and how it can be resolved. Comment: submitted to the ApJ Letters, 10 pages, 3 figures
    The Astrophysical Journal 03/1998; · 6.28 Impact Factor
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    ABSTRACT: We present MMT/Megacam imaging in Sloan g and r of the extremely low luminosity Bootes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and uncertainties on, Bootes II's distance, luminosity, size, and morphology. Comparisons with theoretical isochrones and empirical globular cluster fiducials show that Bootes II's stellar population is old and metal-poor ((Fe/H) ∼ < -2). Assuming a stellar population like that of M92, Bootes II is at a distance of 42 ± 2 kpc, closer than the initial published estimate of 60 ± 10 kpc. This distance revision, combined with a more robust measurement of Bootes II's structure with a Plummer model (exponential model) results in a more compact inferred physical half-light size of rh ≃ 36(33) ±9(10)pc and lower inferred luminosity of MV ≃ −2.4(−2.2) ± 0.7(0.7) mag. The revised size and luminosity we calculate move Bootes II into a region of size-luminosity space not previously known to be occupied by old stellar populations, but also occupied by the recently discovered Milky Way satellites Willman 1 and SEGUE 1. We show that the apparently distorted morphology of Bootes II is not statistically significant given the present data. We use a tidal argument to support a scenario where Bootes II is a dwarf galaxy (dark matter dominated) rather than a globular cluster (not dark matter dominated), although the uncertainty on the M/L we infer for Bootes II is substantial. Moreover, we can not rule out that Bootes II is a star cluster on the verge of disruption, such as Palomar 5. Subject headings: galaxies: dwarf — Local Group
  • J. Harris, D. Zaritsky

Publication Stats

143 Citations
45.54 Total Impact Points

Institutions

  • 2011
    • Paris Diderot University
      Lutetia Parisorum, Île-de-France, France
  • 2006–2008
    • The University of Arizona
      • Department of Astronomy
      Tucson, Arizona, United States
  • 2004
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1998
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States