arXiv subjects

John M. Carpenter

Publications and source records attributed to John M. Carpenter.

At least 109 records · Page 6Linked to original sources

arXiv2009

Debris Disks in the Upper Scorpius OB Association

We present MIPS 24um and 70um photometry for 205 members of the Upper Scorpius OB Association. These data are combined with published MIPS photometry for 15 additional association members to assess the frequency of circumstellar disks around 5 Myr old stars with spectral types between B0 and M5. Twelve stars have a detectable 70um excess, each of which also has a detectable 24um excess. A total of 54 stars are identified with a 24um excess more than 32% above the stellar photosphere. The MIPS observations reveal 19 excess sources -- 8 A/F/G stars and 11 K/M stars -- that were not previously identified with an 8um or 16um excess. The lack of short-wavelength emission and the weak 24um excess suggests that these sources are debris systems or the remnants of optically thick primordial disks with inner holes. Despite the wide range of luminosities of the stars hosting apparent debris systems, the excess characteristics are consistent with all stars having dust at similar orbital radii after factoring in variations in the radiation blowout particle size with spectral type. The results for Upper Sco are compared to similar photometric surveys from the literature to re-evaluate the evolution of debris emission. After considering the completeness limits of published surveys and the effects of stellar evolution on the debris luminosity, we find that the magnitude of the 24um excess around F-type stars increases between ages of 5 and 17 Myr as found by previous studies, but at < 2.6 sigma confidence. For B7-A9 and G0-K5 stars, any variations in the observed 24um excess emission over this age range are significant at less than 2 sigma confidence.

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arXiv2009

Structure and evolution of pre-main sequence circumstellar disks

We present new sub-arcsecond (0.7'') Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of the 1.3 mm continuum emission from circumstellar disks around 11 low and intermediate mass pre-main sequence stars. High resolution observations for 3 additional sources were obtained from literature. In all cases the disk emission is spatially resolved. We adopt a self consistent accretion disk model based on the similarity solution for the disk surface density and constrain the dust radial density distribution on spatial scales of about 40 AU. Disk surface densities appear to be correlated with the stellar ages where the characteristic disk radius increases from ~ 20 AU to 100 AU over about 5 Myr. This disk expansion is accompanied by a decrease in the mass accretion rate, suggesting that our sample disks form an evolutionary sequence. Interpreting our results in terms of the temporal evolution of a viscous $α$-disk, we estimate (i) that at the beginning of the disk evolution about 60% of the circumstellar material was located inside radii of 25--40 AU, (ii) that disks formed with masses from 0.05 to 0.4 M$_{\sun}$ and (iii) that the viscous timescale at the disk initial radius is about 0.1-0.3 Myr. Viscous disk models tightly link the surface density $Σ(R)$ with the radial profile of the disk viscosity $ν(R) \propto R^γ$. We find values of $γ$ ranging from -0.8 to 0.8, suggesting that the viscosity dependence on the orbital radius can be very different in the observed disks. Adopting the $α$ parameterization for the viscosity, we argue that $α$ must decrease with the orbital radius and that it may vary between 0.5 and $10^{-4}$. (abridged)

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arXiv2009

Anachronistic Grain Growth and Global Structure of the Protoplanetary Disk Associated with the Mature Classical T Tauri Star, PDS 66

We present ATCA interferometric observations of the old (13 Myr), nearby (86pc) classical T Tauri star, PDS 66. Unresolved 3 and 12 mm continuum emission is detected towards PDS 66, and upper limits are derived for the 3 and 6 cm flux densities. The mm-wave data show a spectral slope flatter than that expected for ISM-sized dust particles, which is evidence of grain growth. We also present HST/NICMOS 1.1 micron PSF-subtracted coronagraphic imaging of PDS 66. The HST observations reveal a bilaterally symmetric circumstellar region of dust scattering about 0.32% of the central starlight, declining radially in surface brightness. The light-scattering disk of material is inclined 32 degrees from face-on, and extends to a radius of 170 AU. These data are combined with published optical and longer wavelength observations to make qualitative comparisons between the median Taurus and PDS 66 spectral energy distributions (SEDs). By comparing the near-infrared emission to a simple model, we determine that the location of the inner disk radius is consistent with the dust sublimation radius (1400 K at 0.1 AU). We place constraints on the total disk mass using a flat-disk model and find that it is probably too low to form gas giant planets according to current models. Despite the fact that PDS 66 is much older than a typical classical T Tauri star (< 5 Myr), its physical properties are not much different.

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arXiv2009

Spitzer Spectroscopy of Circumstellar Disks in the 5 Myr Old Upper Scorpius OB Association

We present mid-infrared spectra between 5.2 and 38 microns for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 microns. In contrast, 10 and 20 micron silicate features are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral type dependent, with the most prominent features being associated with K5-M2 type stars. By fitting the spectral energy distributions (SED) of a sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high resolution optical spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only 7 (all late-type) exhibit signatures of accretion. Mass accretion rates for these stars range from ~1e-11 to 1e-9 solar masses/yr. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near and mid-infrared excess emission and an order of magnitude lower mass accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time.

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arXiv2008

Formation and Evolution of Planetary Systems (FEPS): Properties of Debris Dust around Solar-type Stars

We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low resolution) observations for 314 stars in the Formation and Evolution of Planetary Systems (FEPS) Legacy program. These data are used to investigate the properties and evolution of circumstellar dust around solar-type stars spanning ages from approximately 3 Myr to 3 Gyr. We identify 46 sources that exhibit excess infrared emission above the stellar photosphere at 24um, and 21 sources with excesses at 70um. Five sources with an infrared excess have characteristics of optically thick primordial disks, while the remaining sources have properties akin to debris systems. The fraction of systems exhibiting a 24um excess greater than 10.2% above the photosphere is 15% for ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the 70um fractional luminosity appears to decline over a similar age range. The characteristic temperature of the debris inferred from the IRS spectra range between 60 and 180 K, with evidence for the presence of cooler dust to account for the strength of the 70um excess emission. No strong correlation is found between dust temperature and stellar age. Comparison of the observational data with disk models containing a power-law distribution of silicate grains suggest that the typical inner disk radius is > 10 AU. Although the interpretation is not unique, the lack of excess emission shortwards of 16um and the relatively flat distribution of the 24um excess for ages <300~Myr is consistent with steady-state collisional models.

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arXiv2008

A Large-Area Search for Low-Mass Objects in Upper Scorpius II: Age and Mass Distributions

We present continued results from a wide-field, ~150 deg^2, optical photometric and spectroscopic survey of the northern part of the ~5 Myr-old Upper Scorpius OB Association. Photometry and spectral types were used to derive effective temperatures and luminosities and place newly identified association members onto a theoretical Hertzsprung-Russell diagram. From our survey, we have discovered 145 new low mass members of the association, and determined ~10% of these objects to be actively accreting material from a surrounding circumstellar disk. Based on comparison of the spatial distributions of low and high mass association members, we find no evidence for spatial segregation by mass within the northern portion of the association. Measured data are combined with pre-main sequence evolutionary models to derive a mass and age for each star. Using Monte Carlo simulations we show that, taking into account known observational uncertainties, the observed age dispersion for the low mass population in USco is consistent with all stars forming in a single burst ~5 Myr ago, and place an upper limit of +/-3 Myr on the age spread if the star formation rate has been constant in time. We derive the first spectroscopic mass function for USco that extends into the substellar regime, and compare these results to those for three other young clusters and associations.

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arXiv2008

A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages

Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6+/-1.6% are younger than 100 Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (Dec < 0 deg) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, beta Pictoris, and AB Doradus. We use a homogeneously observed sample of roughly one hundred and fifty 6300-10000 Angstrom spectra of L and T dwarfs taken with the Low-Resolution Imaging Spectrometer at the W. M. Keck Observatory to examine the strength of the 6708-A Li I line as a function of spectral type and further corroborate the trends noted by Kirkpatrick et al. (2000). We use our low-gravity spectra to investigate the strength of the Li I line as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a few 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmosphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs of well determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat first cited in Kirkpatrick et al. (2006) that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

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arXiv2008

The Formation and Evolution of Planetary Systems: Description of the Spitzer Legacy Science Database

We present the science database produced by the Formation and Evolution of Planetary Systems (FEPS) Spitzer Legacy program. Data reduction and validation procedures for the IRAC, MIPS, and IRS instruments are described in detail. We also derive stellar properties for the FEPS sample from available broad-band photometry and spectral types, and present an algorithm to normalize Kurucz synthetic spectra to optical and near-infrared photometry. The final FEPS data products include IRAC and MIPS photometry for each star in the FEPS sample and calibrated IRS spectra.

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arXiv2008

Molecular Gas in the z=1.2 Ultraluminous Merger GOODS J123634.53+621241.3

We report the detection of CO(2-1) emission from the z=1.2 ultraluminous infrared galaxy (ULIRG) GOODS J123634.53+621241.3 (also known as the sub-millimeter galaxy GN26). These observations represent the first discovery of high-redshift CO emission using the new Combined Array for Research in Millimeter-Wave Astronomy (CARMA). Of all high-redshift (z>1) galaxies within the GOODS-North field, this source has the largest far-infrared (FIR) flux observed in the Spitzer 70um and 160um bands. The CO redshift confirms the optical identification of the source, and the bright CO(2-1) line suggests the presence of a large molecular gas reservoir of about 7x10^10 M(sun). The infrared-to-CO luminosity ratio of L(IR)/L'(CO) = 80+/-30 L(sun) (K Km/s pc^2)^-1 is slightly smaller than the average ratio found in local ULIRGs and high-redshift sub-millimeter galaxies. The short star-formation time scale of about 70 Myr is consistent with a starburst associated with the merger event and is much shorter than the time scales for spiral galaxies and estimates made for high-redshift galaxies selected on the basis of their B-z and z-K colors.

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arXiv2008

Proplyds and Massive Disks in the Orion Nebula Cluster Imaged with CARMA and SMA

[Abridged] We imaged a 2' x 2' region of the Orion Nebula cluster in 1.3 mm wavelength continuum emission with the recently commissioned Combined Array for Research in Millimeter Astronomy (CARMA) and with the Submillimeter Array (SMA). Our mosaics include >250 known near-IR cluster members, of which 36 are so-called "proplyds" that have been imaged previously with the Hubble Space Telescope. We detected 40 sources in 1 mm continuum emission, and several of them are spatially resolved with our observations. Dust masses inferred for detected sources range from 0.01 to 0.5 Msun, and the average disk mass for undetected sources is estimated to be ~0.001 Msun, approximately an order of magnitude smaller than the minimum mass solar nebula. Most stars in the ONC thus do not appear to currently possess sufficient mass in small dust grains to form Jupiter-mass (or larger) planets. Comparison with previous results for younger and older regions indicates that massive disks evolve significantly on ~Myr timescales. We also show that the percentage of stars in Orion surrounded by disks more massive than ~0.01 Msun is substantially lower than in Taurus, indicating that environment has an impact on the disk mass distribution. Finally, we explore potential correlations of disk mass with stellar mass and location within the cluster.

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arXiv2007

The Complete Census of 70-um-Bright Debris Disks within the FEPS (Formation and Evolution of Planetary Systems) Spitzer Legacy Survey of Sun-like Stars

(abbreviated) We report detection with the Spitzer Space Telescope of cool dust surrounding solar type stars. The observations were performed as part of the Legacy Science Program, ``Formation and Evolution of Planetary Systems'' (FEPS). From the overall FEPS sample (Meyer et al. 2006) of 328 stars having ages ~0.003-3 Gyr we have selected sources with 70 um flux densities indicating excess in their spectral energy distributions above expected photospheric emission........ .....The rising spectral energy distributions towards - and perhaps beyond - 70 um imply dust temperatures T_dust <45-85 K for debris in equilibrium with the stellar radiation field. We infer bulk properties such as characteristic temperature, location, fractional luminosity, and mass of the dust from fitted single temperature blackbody models. For >1/3 of the debris sources we find that multiple temperature components are suggested, implying a spatial distribution of dust extending over many tens of AU. Because the disks are dominated by collisional processes, the parent body (planetesimal) belts may be extended as well. Preliminary assessment of the statistics of cold debris around sun-like stars shows that ~10% of FEPS targets with masses between 0.6 and 1.8 Msun and ages between 30 Myr and 3 Gyr exhibit 70 um emission in excess of the expected photospheric flux density. We find that fractional excess amplitudes appear higher for younger stars and that there may be a trend in 70 um excess frequency with stellar mass.

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arXiv2007

The dust, planetesimals and planets of HD 38529

HD 38529 is a post-main sequence G8III/IV star (3.5 Gyr old) with a planetary system consisting of at least two planets having Msin(i) of 0.8 MJup and 12.2 MJup, semimajor axes of 0.13 AU and 3.74 AU, and eccentricities of 0.25 and 0.35, respectively. Spitzer observations show that HD 38529 has an excess emission above the stellar photosphere, with a signal-to-noise ratio (S/N) at 70 micron of 4.7, a small excess at 33 micron (S/N=2.6) and no excess <30 micron. We discuss the distribution of the potential dust-producing planetesimals from the study of the dynamical perturbations of the two known planets, considering in particular the effect of secular resonances. We identify three dynamically stable niches at 0.4-0.8 AU, 20-50 AU and beyond 60 AU. We model the spectral energy distribution of HD 38529 to find out which of these niches show signs of harboring dust-producing plantesimals. The secular analysis, together with the SED modeling resuls, suggest that the planetesimals responsible for most of the dust emission are likely located within 20-50 AU, a configuration that resembles that of the Jovian planets + Kuiper Belt in our Solar System. Finally, we place upper limits (8E-6 lunar masses of 10 micron particles) to the amount of dust that could be located in the dynamically stable region that exists between the two planets (0.25--0.75 AU).

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arXiv2006

Are Debris Disks and Massive Planets Correlated?

Using data from the Spitzer Space Telescope Legacy Science Program ``Formation and Evolution of Planetary Systems'' (FEPS), we have searched for debris disks around 9 FGK stars (2-10 Gyr), known from radial velocity (RV) studies to have one or more massive planets. Only one of the sources, HD 38529, has excess emission above the stellar photosphere; at 70 micron the signal-to-noise ratio in the excess is 4.7 while at wavelengths < 30 micron there is no evidence of excess. The remaining sources show no excesses at any Spitzer wavelengths. Applying survival tests to the FEPS sample and the results for the FGK survey published in Bryden et al. (2006), we do not find a significant correlation between the frequency and properties of debris disks and the presence of close-in planets. We discuss possible reasons for the lack of a correlation.

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arXiv2006

Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr-old Upper Scorpius OB Association

We present 4.5, 8, and 16um photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB Association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between ~ 0.1 and 20 Msun at an age of ~ 5 Myr. We identify 35 stars that have emission at 8um or 16um in excess of the stellar photosphere. The lower mass stars (~ 0.1-1.2 Msun) appear surrounded by primordial optically thick disks based on the excess emission characteristics. Stars more massive than ~ 1.8 Msun have lower fractional excess luminosities suggesting that the inner ~ 10 AU of the disk has been largely depleted of primordial material. None of the G and F stars (~ 1.2-1.8 Msun) in our sample have an infrared excess at wavelengths <= 16um. These results indicate that the mechanisms for dispersing primordial optically thick disks operate less efficiently on average for low mass stars, and that longer time scales are available for the buildup of planetary systems in the terrestrial zone for stars with masses < 1 Msun.

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arXiv2006

A Distributed Population of Low Mass Pre-Main Sequence Stars near the Taurus Molecular Clouds

We present a drift scan survey covering a ~5 deg by 50 deg region toward the southern portion of the Taurus-Auriga molecular cloud. Data taken in the B,R,I filters with the Quest-2 camera on the Palomar 48-inch telescope were combined with 2MASS near-infrared photometry to select candidate young stars. Follow-up optical spectroscopy of 190 candidates led to identification of 42 new low mass pre-main sequence stars with spectral types M4-M8, of which approximately half exhibit surface gravity signatures similar to known Taurus stars while the other half exhibit surface gravity signatures similar to members of the somewhat older Upper Sco, TW Hya and Beta Pic associations. The pre-main sequence stars are spread over ~35 deg, and many are located well outside of previously explored regions. From assessment of the spatial and proper motion distributions, we argue that the new pre-main sequence stars identified far from the clouds cannot have originated from the vicinity of the 1-2 Myr-old subclusters which contain the bulk of the identified Taurus members, but instead represent a newly-identified area of recent star-formation near the clouds.

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arXiv2006

Massive Protoplanetary Disks in the Trapezium Region

(abridged) We determine the disk mass distribution around 336 stars in the young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum emission with the Owens Valley Millimeter Array. For this sample of 336 stars, we observe 3 mm emission above the 3-sigma noise level toward ten sources, six of which have also been detected optically in silhouette against the bright nebular background. In addition, we detect 20 objects that do not correspond to known near-IR cluster members. Comparisons of our measured fluxes with longer wavelength observations enable rough separation of dust emission from thermal free-free emission, and we find substantial dust emission toward most objects. For the ten objects detected at both 3 mm and near-IR wavelengths, eight exhibit substantial dust emission. Excluding the high-mass stars and assuming a gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to 0.39 Msun. For the cluster members not detected at 3 mm, images of individual objects are stacked to constrain the mean 3 mm flux of the ensemble. The average flux is detected at the 3-sigma confidence level, and implies an average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula. The percentage of stars in Orion surrounded by disks more massive than ~0.1 Msun is consistent with the disk mass distribution in Taurus, and we argue that massive disks in Orion do not appear to be truncated through close encounters with high-mass stars. Comparison of the average disk mass and number of massive dusty structures in Orion with similar surveys of the NGC 2024 and IC 348 clusters constrains the evolutionary timescales of massive circumstellar disks in clustered environments.

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arXiv2005

Constraining the Evolutionary Stage of Class I Protostars: Multi-wavelength Observations and Modeling

(abridged) We present new Keck images at 0.9 micron and OVRO 1.3 mm continuum images of five Class I protostars in the Taurus star forming region. We analyze these data in conjunction with broadband spectral energy distributions and 8-13 micron spectra from the literature using a Monte Carlo radiative transfer code. By fitting models for the circumstellar dust distributions simultaneously to the scattered light images, millimeter continuum data, and the SEDs, we attempt to distinguish between flared disks, infalling envelopes with outflow cavities, and combinations of disks and envelopes. For each of these circumstellar density distributions, we generate grids of models for varying geometries, dust masses, and accretion rates, and determine the best fits by minimizing the residuals between model and data. Models incorporating both massive envelopes and massive embedded disks generally fit the imaging+SED data best. Implied envelope infall rates for these models are consistent with infall rates derived by previous investigators, although they are approximately an order of magnitude larger than inner disk accretion rates inferred from recent spectroscopic measurements. In addition, the disk masses inferred from our models are close to or larger than the limit for gravitationally stable disks, indicating that Class I disks may undergo periodic episodes of enhanced accretion, perhaps as a result of gravitational instabilities. An important caveat to these results is that in some cases, no single model can fit all of the imaging and SED data well, suggesting that further refinements to models of the circumstellar dust distributions around Class I sources are necessary.

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arXiv2005

Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun-like stars

We present the discovery of debris systems around three solar mass stars based upon observations performed with the Spitzer Space Telescope as part of a Legacy Science Program, ``the Formation and Evolution of Planetary Systems'' (FEPS). We also confirm the presence of debris around two other stars. All the stars exhibit infrared emission in excess of the expected photospheres in the 70 micron band, but are consistent with photospheric emission at <= 33 micron. This restricts the maximum temperature of debris in equilibrium with the stellar radiation to T < 70 K. We find that these sources are relatively old in the FEPS sample, in the age range 0.7 - 3 Gyr. Based on models of the spectral energy distributions, we suggest that these debris systems represent materials generated by collisions of planetesimal belts. We speculate on the nature of these systems through comparisons to our own Kuiper Belt, and on the likely planet(s) responsible for stirring the system and ultimately releasing dust through collisions. We further report observations of a nearby star HD 13974 (d =11 pc) that is indistinguishable from a bare photosphere at both 24 micron and 70 micron. The observations place strong upper limits on the presence of any cold dust in this nearby system (L_IR/L_* < 10^{-5.2}).

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