Papers by Deborah Padgett
Publications of the Astronomical Society of the Pacific, 2003
Crucial steps in the formation of stars and planets can be studied only at mid-to far-infrared wa... more Crucial steps in the formation of stars and planets can be studied only at mid-to far-infrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and starforming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6-70 mm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planet-forming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg 2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from ground-based telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many follow-up studies with ground-based telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin.
We present Spitzer 3.6 and 4.5 µm photometry and positions for a sample of 1510 brown dwarf candi... more We present Spitzer 3.6 and 4.5 µm photometry and positions for a sample of 1510 brown dwarf candidates identified by the WISE All-Sky Survey. Of these, 166 have been spectroscopically classified as objects with spectral types M(1), L(7), T(146), and Y(12); Sixteen other objects are non-(sub)stellar in nature. The remainder are most likely distant L and T dwarfs lacking spectroscopic verification, other Y dwarf candidates still awaiting follow-up, and
We discuss optical associations, spectral energy distributions and photometric redshifts for SWIR... more We discuss optical associations, spectral energy distributions and photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 µm) and MIPS (24, 70, 160 µm) data have been associated with optical UgriZ data from the INT Wide Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. Criteria for eliminating spurious
This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Ext... more This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Extragalactic (SWIRE) fields. The source counts are compared to counts in other fields, and to model predictions that have been updated since the launch of Spitzer. This analysis confirms a very steep rise in the Euclidean-normalized differential number counts between 2 mJy and 0.3 mJy. Variations in the counts between fields show the effects of sample variance in the flux range 0.5-10 mJy, up to 100% larger than Poisson errors. Nonetheless, a “shoulder ” in the normalized counts persists at around 3 mJy. The peak of the normalized counts at 0.3 mJy is higher and narrower than most models predict. In the ELAIS N1 field, the 24 micron data are combined with Spitzer-IRAC data and five-band optical imaging, and these bandmerged data are fit with photometric redshift templates. Above 1 mJy the counts are dominated by galaxies at z < 0.3. By 300 microJy, about 25 % are between z ∼0.3-0.8, and...
We characterize the SWIRE galaxy populations in the SWIRE validation field within the Lockman Hol... more We characterize the SWIRE galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24µ Spitzer data and deep U,g ′,r ′,i ′ optical imaging within an area ∼1/3 sq. deg for ∼16,000 Spitzer-SWIRE sources.
arXiv: Solar and Stellar Astrophysics, 2019
The majority of the ultimate main-sequence mass of a star is assembled in the protostellar phase,... more The majority of the ultimate main-sequence mass of a star is assembled in the protostellar phase, where a forming star is embedded in an infalling envelope and encircled by a protoplanetary disk. Studying mass accretion in protostars is thus a key to understanding how stars gain their mass and ultimately how their disks and planets form and evolve. At this early stage, the dense envelope reprocesses most of the luminosity generated by accretion to far-infrared and submillimeter wavelengths. Time-domain photometry at these wavelengths is needed to probe the physics of accretion onto protostars, but variability studies have so far been limited, in large part because of the difficulty in accessing these wavelengths from the ground. We discuss the scientific progress that would be enabled with far-infrared and submillimeter programs to probe protostellar variability in the nearest kiloparsec.
The Astrophysical Journal Supplement Series, 2019
We study a very young star-forming region in the outer Galaxy that is the most concentrated sourc... more We study a very young star-forming region in the outer Galaxy that is the most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360 survey. This region, dubbed CMa-l224, is located in the Canis Major OB1 association. CMa-l224 is relatively faint in the mid-infrared, but it shines brightly at the far-infrared wavelengths as revealed by the Herschel Space Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 µm data from the Spitzer/GLIMPSE360 survey, combined with the JHK s 2MASS and the 70-500 µm Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection criteria based on color-color cuts and fitting of the YSO candidates' spectral energy distributions with YSO 2D radiative transfer models. We identify 293 YSO candidates and estimate physical parameters for 210 sources well-fit with YSO models. We select an additional 47 sources with GLIMPSE360only photometry as 'possible YSO candidates'. The vast majority of these sources are associated with high H 2 column density regions and are good targets for follow-up studies. The distribution of YSO candidates at different evolutionary stages with respect to Herschel filaments supports the idea that stars are formed in the filaments and become more dispersed with time. Both the supernova-induced and spontaneous star formation scenarios are plausible in the environmental context of CMa-l224. However, our results indicate that a spontaneous gravitational collapse of filaments is a more likely scenario. The methods developed for CMa-l224 can be used for larger regions in the Galactic plane where the same set of photometry is available.
The Astronomical Journal, 2017
We describe a joint high-contrast imaging survey for planets at the Keck and Very Large Telescope... more We describe a joint high-contrast imaging survey for planets at the Keck and Very Large Telescope of the last large sample of debris disks identified by the Spitzer Space Telescope. No new substellar companions were discovered in our survey of 30 Spitzer-selected targets. We combine our observations with data from four published surveys to place constraints on the frequency of planets around 130 debris disk single stars, the largest sample to date. For a control sample, we assembled contrast curves from several published surveys targeting 277 stars that do not show infrared excesses. We assumed a double power-law distribution in mass and semimajor axis (SMA) of the form f m a Cm a , = a b () , where we adopted power-law values and logarithmically flat values for the mass and SMA of planets. We find that the frequency of giant planets with masses 5-20 M Jup and separations 10-1000 au around stars with debris disks is 6.27% (68% confidence interval 3.68%-9.76%), compared to 0.73% (68% confidence interval 0.20%-1.80%) for the control sample of stars without disks. These distributions differ at the 88% confidence level, tentatively suggesting distinctness of these samples.
The Astronomical Journal, 2000
In a previous paper we proposed that there may be a population of runaway planets and brown dwarf... more In a previous paper we proposed that there may be a population of runaway planets and brown dwarfs that formed via ejection from multiple-star systems. We further suggested TMR-1C as a candidate runaway protoplanet. Hubble Space T elescope NICMOS images of the Class I protostar TMR-1 (IRAS 04361]2547) reveal TMR-1C as a faint near-infrared companion with AU projected 10A .0 \ 1400 separation. The central protostar is itself resolved as a close binary with AU separation, sur-0A .31 \ 42 rounded by circumstellar reÑection nebulosity. A long, narrow Ðlament seems to connect the protobinary to the faint companion TMR-1C, suggesting a physical association, which can plausibly be explained if TMR-1C was ejected by the protobinary. This paper presents near-infrared grism spectroscopy to constrain the e †ective temperature of TMR-1C, obtained with the Near-Infrared Camera (NIRC) at Keck Observatory. To interpret the data, we construct a grid of extincted M dwarf spectra to compare with the low-resolution (R D 120) NIRC spectrum. The assumed extinction corresponds to standard interstellar dust. With the additional assumption that no near-infrared dust excess contributes to the spectrum, then M4.5 is the latest spectral type TMR-1C can have within the uncertainties. Adopting 2 p error bars, this translates to K e †ective temperature and extinction at K T eff [ 2700 A K \ 2.5^0.75 band for standard dust). We compare the luminosity and e †ective temperature of (A V \ 22^6.6 TMR-1C with evolutionary tracks of young giant planets and brown dwarfs in a theoretical H-R diagram. Given a relatively low inferred luminosity of D10~3 then TMR-1C is hotter than predicted L _ , by available theoretical models. However, the models are very uncertain at such young ages, less than 300,000 yr, so that it is unclear whether the theoretical tracks by themselves provide a suitably strong test. Given the quality of the observed spectrum, only a partial answer is possible. The new data do not lend weight to the protoplanet interpretation, and the results remain consistent with the explanation that TMR-1C may be a background star. We discuss additional observational tests that may be useful.
Astronomy & Astrophysics, 2014
Dust in debris disks is produced by colliding or evaporating planetesimals, remnants of the plane... more Dust in debris disks is produced by colliding or evaporating planetesimals, remnants of the planet formation process. Warm dust disks, known by their emission at 24 µm, are rare (4% of FGK main sequence stars) and especially interesting because they trace material in the region likely to host terrestrial planets, where the dust has a very short dynamical lifetime. Statistical analyses of the source counts of excesses as found with the mid-IR Wide Field Infrared Survey Explorer (WISE) suggest that warm-dust candidates found for the Kepler transiting-planet host-star candidates can be explained by extragalactic or galactic background emission aligned by chance with the target stars. These statistical analyses do not exclude the possibility that a given WISE excess could be due to a transient dust population associated with the target. Here we report Herschel/PACS 100 and 160 micron follow-up observations of a sample of Kepler and non-Kepler transiting-planet candidates' host stars, with candidate WISE warm debris disks, aimed at detecting a possible cold debris disk in any of them. No clear detections were found in any one of the objects at either wavelength. Our upper limits confirm that most objects in the sample do not have a massive debris disk like that in β Pic. We also show that the planet-hosting star WASP-33 does not have a debris disk comparable to the one around η Crv. Although the data cannot be used to rule out rare warm disks around the Kepler planet-hosting candidates, the lack of detections and the characteristics of neighboring emission found at far-IR wavelengths support an earlier result suggesting that most of the WISE-selected IR excesses around Kepler candidate host stars are likely due to either chance alignment with background IR-bright galaxies and/or to interstellar emission.
The Astrophysical Journal, 2011
We have carried out a survey of the north and south ecliptic poles, EP-N and EP-S, respectively, ... more We have carried out a survey of the north and south ecliptic poles, EP-N and EP-S, respectively, with the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE). The primary objective was to cross-calibrate WISE with the Spitzer and Midcourse Space Experiment (MSX) photometric systems by developing a set of calibration stars that are common to these infrared missions. The ecliptic poles were continuous viewing zones for WISE due to its polar-crossing orbit, making these areas ideal for both absolute and internal calibrations. The Spitzer IRAC and MIPS imaging survey covers a complete area of 0.40 deg 2 for the EP-N and 1.28 deg 2 for the EP-S. WISE observed the whole sky in four mid-infrared bands, 3.4, 4.6, 12, and 22 μm, during its eight-month cryogenic mission, including several hundred ecliptic polar passages; here we report on the highest coverage depths achieved by WISE, an area of ∼1.5 deg 2 for both poles. Located close to the center of the EP-N, the Sy-2 galaxy NGC 6552 conveniently functions as a standard calibrator to measure the red response of the 22 μm channel of WISE. Observations from Spitzer-IRAC/MIPS/IRS-LL and WISE show that the galaxy has a strong red color in the mid-infrared due to star-formation and the presence of an active galactic nucleus (AGN), while over a baseline >1 year the mid-IR photometry of NGC 6552 is shown to vary at a level less than 2%. Combining NGC 6552 with the standard calibrator stars, the achieved photometric accuracy of the WISE calibration, relative to the Spitzer and MSX systems, is 2.4%, 2.8%, 4.5%, and 5.7% for W1 (3.4 μm), W2 (4.6 μm), W3 (12 μm), and W4 (22 μm), respectively. The WISE photometry is internally stable to better than 0.1% over the cryogenic lifetime of the mission. The secondary objective of the Spitzer-WISE Survey was to explore the poles at greater flux-level depths, exploiting the higher angular resolution Spitzer observations and the exceptionally deep (in total coverage) WISE observations that potentially reach down to the confusion limit of the survey. The rich Spitzer and WISE data sets were used to study the Galactic and extragalactic populations through source counts, color-magnitude and color-color diagrams. As an example of what the data sets facilitate, we have separated stars from galaxies, delineated normal galaxies from power-law-dominated AGNs, and reported on the different fractions of extragalactic populations. In the EP-N, we find an AGN source density of ∼260 deg −2 to a 12 μm depth of 115 μJy, representing 15% of the total extragalactic population to this depth, similar to what has been observed for low-luminosity AGNs in other fields.
The Astrophysical Journal, 1996
High-resolution echelle spectra have been obtained for low-mass preÈmain-sequence stars in the Ta... more High-resolution echelle spectra have been obtained for low-mass preÈmain-sequence stars in the Taurus-Auriga, Orion, Chamaeleon, Ophiuchus, and other star formation regions. Using temperaturesensitive individual metallic line ratios, new e †ective temperatures are determined for a sample of 30 G and K preÈmain-sequence stars in nearby star-forming clouds to an accuracy of^200 K or better in most cases. Comparison of these values with previous spectral classiÐcations using low-resolution spectra reveals occasional large discrepancies in spectral type. Microturbulences derived for 23 "" weak line ÏÏ T Tauri stars range as high as 3.1^0.4 km s~1. The mean microturbulent velocity of the sample is 1.6^0.6 km s~1. Using these new e †ective temperatures and microturbulences, iron abundances have been determined for 30 preÈmain-sequence stars in several northern and southern sky star-formation regions. By determining the mean [Fe/H] for at least Ðve stars in each cloud, bulk metallicities have been derived for the Taurus-Auriga, Orion, Chamaeleon, and Ophiuchus molecular clouds. [Fe/H] is approximately solar in all the regions surveyed.
The Astrophysical Journal, 2003
We present a model of the circumstellar environment of the so-called "Butterfly Star" in Taurus (... more We present a model of the circumstellar environment of the so-called "Butterfly Star" in Taurus (IRAS 04302+2247). The appearance of this young stellar object is dominated by a large circumstellar disk seen edge-on and the light scattering lobes above the disk. Our model is based on multi-wavelength continuum observations: (1) Millimeter maps, and (2) High-resolution near-infrared obtained with HST/NICMOS. The advantage of the combination of both observations is that they trace (a) different regions of the system and (b) different physical processes. On the one hand, the millimeter-observations are sensitive to the long-wavelength radiation being re-emitted from the dust in the central parts close to the midplane of the circumstellar disk. Thus, the geometry and small-scale density distribution of the disk has been studied. Furthermore, in contrast to the pure flux measurement, the resolved 1.3 mm image allows to discriminate between different disk models with a similar far-infrared/millimeter spectral energy distribution and therefore to disentangle the disk geometry much more precisely. On the other hand, the near-infrared observations trace the envelope structure and dust properties in the envelope and the disk surface. We find disk and envelope parameters which are comparable with those of the circumstellar environment of other young stellar objects. A main result is that the dust properties must be different in the circumstellar disk and in the envelope: While a grain size distribution with grain radii up to 100 µm is required to reproduce the millimeter observations of the disk, the envelope is dominated by smaller grains similar to those of the interstellar medium. Alternatives to this grain growth scenario in the circumstellar disk are discussed in brief as well.
The Astronomical Journal, 2008
This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Ext... more This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Extragalactic (SWIRE) fields. The source counts are compared to counts in other fields, and to model predictions that have been updated since the launch of Spitzer. This analysis confirms a very steep rise in the Euclidean-normalized differential number counts between 2 mJy and 0.3 mJy. Variations in the counts between fields show the effects of sample variance in the flux range 0.5-10 mJy, up to 100% larger than Poisson errors. Nonetheless, a "shoulder" in the normalized counts persists at around 3 mJy. The peak of the normalized counts at 0.3 mJy is higher and narrower than most models predict. In the ELAIS N1 field, the 24 micron data are combined with Spitzer-IRAC data and five-band optical imaging, and these bandmerged data are fit with photometric redshift templates. Above 1 mJy the counts are dominated by galaxies at z < 0.3. By 300 microJy, about 25% are between z ∼0.3-0.8, and a significant fraction are at z ∼1.3-2. At low redshifts the counts are dominated by spirals, and starbursts rise in number density to outnumber the spirals' contribution to the counts below 1 mJy.
Publications of the Astronomical Society of the Pacific, 2005
We describe the data reduction algorithms for the Multiband Imaging Photometer for Spitzer (MIPS)... more We describe the data reduction algorithms for the Multiband Imaging Photometer for Spitzer (MIPS) instrument. These algorithms were based on extensive preflight testing and modeling of the Si:As (24 µm) and Ge:Ga (70 and 160 µm) arrays in MIPS and have been refined based on initial flight data. The behaviors we describe are typical of state-of-the-art infrared focal planes operated in the low backgrounds of space. The Ge arrays are bulk photoconductors and therefore show a variety of artifacts that must be removed to calibrate the data. The Si array, while better behaved than the Ge arrays, does show a handful of artifacts that also must be removed to calibrate the data. The data reduction to remove these effects is divided into three parts. The first part converts the nondestructively read data ramps into slopes while removing artifacts with time constants of the order of the exposure time. The second part calibrates the slope measurements while removing artifacts with time constants longer than the exposure time. The third part uses the redundancy inherit in the MIPS observing modes to improve the artifact removal iteratively. For each of these steps, we illustrate the relevant laboratory experiments or theoretical arguments along with the mathematical approaches taken to calibrate the data. Finally, we describe how these preflight algorithms have performed on actual flight data.
Publications of the Astronomical Society of the Pacific, 2007
We describe the absolute calibration of the Multiband Imaging Photometer for Spitzer (MIPS) 160 µ... more We describe the absolute calibration of the Multiband Imaging Photometer for Spitzer (MIPS) 160 µm channel. After the on-orbit discovery of a near-IR ghost image that dominates the signal for sources hotter than about 2000 K, we adopted a strategy utilizing asteroids to transfer the absolute calibrations of the MIPS 24 and 70 µm channels to the 160 µm channel. Near-simultaneous observations at all three wavelengths are taken, and photometry at the two shorter wavelengths is fit using the Standard Thermal Model. The 160 µm flux density is predicted from those fits and compared with the observed 160 µm signal to derive the conversion from instrumental units to surface brightness. The calibration factor we derive is 41.7 MJy/sr/MIPS160 (MIPS160 being the instrumental units). The scatter in the individual measurements of the calibration factor, as well as an assesment of the external uncertainties inherent in the calibration, lead us to adopt an uncertainty of 5.0 MJy/sr/MIPS160 (12%) for the absolute uncertainty on the 160 µm flux density of a particular source as determined from a single measurement. For sources brighter than about 2 Jy, non-linearity in the response of the 160 µm detectors produces an underestimate of the flux density:
The Astrophysical Journal, 2010
We present 49 new candidate debris disks that were detected around nearby stars with the Spitzer ... more We present 49 new candidate debris disks that were detected around nearby stars with the Spitzer Space Telescope using the Multiband Imaging Photometer (MIPS) at 24 μm (MIPS24) and 70 μm (MIPS70). The survey sample was composed of stars within 25 pc of the Sun that were not previously observed by any other MIPS survey. Only stars with V < 9 were selected, corresponding to spectral types earlier than M0. MIPS24 integration times were chosen to detect the stellar photosphere at 10σ levels or better. MIPS70 observations were designed to detect excess infrared emission from any star in the MIPS70 sample with a disk as luminous at that around Eridani. The resulting sample included over 436 nearby stars that were observed with both MIPS24 and MIPS70, plus an additional 198 observed only with MIPS24. Debris disk candidates were defined as targets where excess emission was detected at 3σ levels or greater, and the ratio of observed flux density to expected photosphere emission was three standard deviations or more above the mean value for the sample. The detection rate implied by the resulting 29 MIPS24 candidates is 4.6%. A detection rate of 4.8% is implied by 21 MIPS70 candidates. The distribution of spectral types for stars identified as candidates resembles that of the general sample and yields strong evidence that debris-disk occurrence does not decrease for K dwarfs. Modeling of non-uniform sensitivity in the sample is required to interpret quantitative estimates of the overall detection frequency and will be presented in a future work.
The last two decades have seen remarkable progress in our long-standing goal of determining the a... more The last two decades have seen remarkable progress in our long-standing goal of determining the abundance and diversity of worlds in the Galaxy. Understanding of this subject involves tracing the path of interstellar material from dense cloud cores, to young stellar objects, protoplanetary disks, and finally extrasolar planets. Here we discuss the critical information provided on these objects by point-source far-ultraviolet spectroscopy with a large aperture, high resolution spectrograph of a large sample of unique protostellar and protoplanetary objects that will leverage our existing knowledge to lay out a path to new and powerful insight into the formation process. We lay out a systematic case of coordinated observations that will yield new knowledge about the process of assembly for both protostellar and protoplanetary systems-that addresses specific uncertainties in our current knowledge and takes advantage of potential new technologies to acquire the data needed.
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Papers by Deborah Padgett