27,307 research outputs found
Are debris disks self-stirred?
This paper considers the evidence that debris disks are self-stirred by the
formation of Plutos. A model for the dust produced during self-stirring is
applied to statistics for A stars. As there is no significant difference
between excesses of A-stars <50Myr old, we focus on reproducing the broad
trends, the "rise and fall" of the fraction of stars with excesses. Using a
population model, we find that the statistics and trends can be reproduced with
a self-stirring model of planetesimal belts with radii distributed between
15-120AU. Disks must have this 15AU minimum radius to show a peak in disk
fraction, rather than a monotonic decline. Populations of extended disks with
fixed inner and/or outer radii fail to fit the statistics, due mainly to the
slow 70um evolution as stirring moves further out in the disk. This conclusion,
that debris disks are narrow belts, is independent of the significance of 24um
trends for young A-stars. We show that the statistics can also be reproduced
with a model in which disks are stirred by secular perturbations from a nearby
eccentric planet. Detailed imaging is therefore the best way to characterise
the stirring mechanism. From a more detailed look at beta Pictoris Moving Group
and TW Hydrae Association A-stars we find that the disk around beta Pictoris is
likely the result of secular stirring by the proposed planet at ~10AU; the
structure of the HR 4796A disk also points to sculpting by a planet. The two
other stars with disks, HR 7012 and eta Tel, possess transient hot dust, though
the outer eta Tel disk is consistent with a self-stirred origin. Planet
formation provides a natural explanation for the belt-like nature of debris
disks, with inner regions cleared by planets that may also stir the disk, and
the outer edges set by where planetesimals can form. [abridged]Comment: Accepted to MNRA
The Herschel Digit Survey Of Weak-Line T Tauri Stars: Implications For Disk Evolution And Dissipation
As part of the "Dust, Ice, and Gas In Time (DIGIT)" Herschel Open Time Key Program, we present Herschel photometry (at 70, 160, 250, 350, and 500 mu m) of 31 weak-line T Tauri star (WTTS) candidates in order to investigate the evolutionary status of their circumstellar disks. Of the stars in our sample, 13 had circumstellar disks previously known from infrared observations at shorter wavelengths, while 18 of them had no previous evidence for a disk. We detect a total of 15 disks as all previously known disks are detected at one or more Herschel wavelengths and two additional disks are identified for the first time. The spectral energy distributions (SEDs) of our targets seem to trace the dissipation of the primordial disk and the transition to the debris disk regime. Of the 15 disks, 7 appear to be optically thick primordial disks, including 2 objects with SEDs indistinguishable from those of typical Classical T Tauri stars, 4 objects that have significant deficit of excess emission at all IR wavelengths, and 1 "pre-transitional" object with a known gap in the disk. Despite their previous WTTS classification, we find that the seven targets in our sample with optically thick disks show evidence for accretion. The remaining eight disks have weaker IR excesses similar to those of optically thin debris disks. Six of them are warm and show significant 24 mu m Spitzer excesses, while the last two are newly identified cold debris-like disks with photospheric 24 mu m fluxes, but significant excess emission at longer wavelengths. The Herschel photometry also places strong constraints on the non-detections, where systems with F-70/F-70,(*) greater than or similar to 5-15 and L-disk/L-* greater than or similar to 10(-3) to 10(-4) can be ruled out. We present preliminary models for both the optically thick and optically thin disks and discuss our results in the context of the evolution and dissipation of circumstellar disks.NASA through JPL/CaltechNASA through the Sagan Fellowship ProgramEuropean Commission PERG06-GA-2009-256513Agence Nationale pour la Recherche (ANR) of France ANR-2010-JCJC-0504-01CFHT 11AH96Astronom
The ACS Fornax Cluster Survey. II. The Central Brightness Profiles of Early-Type Galaxies: A Characteristic Radius on Nuclear Scales and the Transition from Central Luminosity Deficit to Excess
We analyse HST surface brightness profiles for 143 early-type galaxies in the
Virgo and Fornax Clusters. Sersic models provide accurate descriptions of the
global profiles with a notable exception: the observed profiles deviate
systematically inside a characteristic "break" radius of R_b ~ 0.02R_e where
R_e is the effective radius of the galaxy. The sense of the deviation is such
that bright galaxies (M_B < -20) typically show central light deficits with
respect to the inward extrapolation of the Sersic model, while the great
majority of low- and intermediate-luminosity galaxies (-19.5 < M_B < -15) show
central light excesses; galaxies occupying a narrow range of intermediate
luminosities (-20 < M_B < -19.5) are usually well fitted by Sersic models over
all radii. The slopes of the central surface brightness profiles, when measured
at fixed fractions of R_e, vary smoothly as a function of galaxy luminosity in
a manner that depends sensitively on the choice of measurement radius. We show
that a recent claim of strong bimodality in slope is likely an artifact of the
galaxy selection function used in that study. To provide a more robust
characterization of the inner regions of galaxies, we introduce a parameter
that describes the central luminosity deficit or excess relative to the inward
extrapolation of the outer Sersic model. We find that this parameter varies
smoothly over the range of ~ 720 in blue luminosity spanned by the Virgo and
Fornax sample galaxies, with no evidence for a dichotomy. We argue that the
central light excesses (nuclei) in M_B > -19 galaxies may be the analogs of the
dense central cores that are predicted by some numerical simulations to form
via gas inflows. (ABRIDGED)Comment: To appear in the Astrophysical Journal, December 2007. Full
resolution paper available at
http://www.cfa.harvard.edu/~ajordan/ACSFCS/Home.htm
Ten-Micron Observations of Nearby Young Stars
(abridged) We present new 10-micron photometry of 21 nearby young stars
obtained at the Palomar 5-meter and at the Keck I 10-meter telescopes as part
of a program to search for dust in the habitable zone of young stars. Thirteen
of the stars are in the F-K spectral type range ("solar analogs"), 4 have B or
A spectral types, and 4 have spectral type M. We confirm existing IRAS
12-micron and ground-based 10-micron photometry for 10 of the stars, and
present new insight into this spectral regime for the rest. Excess emission at
10 micron is not found in any of the young solar analogs, except for a possible
2.4-sigma detection in the G5V star HD 88638. The G2V star HD 107146, which
does not display a 10-micron excess, is identified as a new Vega-like
candidate, based on our 10-micron photospheric detection, combined with
previously unidentified 60-micron and 100-micron IRAS excesses. Among the
early-type stars, a 10-micron excess is detected only in HD 109573A (HR 4796A),
confirming prior observations; among the M dwarfs, excesses are confirmed in AA
Tau, CD -40 8434, and Hen 3-600A. A previously suggested N band excess in the
M3 dwarf CD -33 7795 is shown to be consistent with photospheric emission.Comment: 40 pages, 4 figures, 5 tables. To appear in the January 1, 2004 issue
of Ap
Varieties of Currency Crises
The plethora of currency crises around the world has fueled many theories on the causes of speculative attacks. The first-generation models focus on fiscal problems. The second-generation models emphasize countercyclical policies and self-fulfilling crises. In the 1990s, models pinpoint to financial excesses. With the crisis of Argentina in 2001, models of sovereign default have become popular again. While the theoretical literature has emphasized variety, the empirical literature has supported the one size fits all' models. This paper contributes to the empirical literature by assessing whether the crises of the last thirty years are of different varieties. Crises are found to be of six varieties. Four of those varieties are associated with domestic economic fragility. But crises can also be provoked by just adverse world market conditions, such as the reversal of international capital flows. The so-called sudden-stop phenomenon identifies the fifth variety of crises. Finally, a small number of crises occur in economies with immaculate fundamentals but this type of crises is not an emerging-market phenomenon.
Radiation-Hydrodynamics of Hot Jupiter Atmospheres
Radiative transfer in planetary atmospheres is usually treated in the static
limit, i.e., neglecting atmospheric motions. We argue that hot Jupiter
atmospheres, with possibly fast (sonic) wind speeds, may require a more
strongly coupled treatment, formally in the regime of radiation-hydrodynamics.
To lowest order in v/c, relativistic Doppler shifts distort line profiles along
optical paths with finite wind velocity gradients. This leads to flow-dependent
deviations in the effective emission and absorption properties of the
atmospheric medium. Evaluating the overall impact of these distortions on the
radiative structure of a dynamic atmosphere is non-trivial. We present
transmissivity and systematic equivalent width excess calculations which
suggest possibly important consequences for radiation transport in hot Jupiter
atmospheres. If winds are fast and bulk Doppler shifts are indeed important for
the global radiative balance, accurate modeling and reliable data
interpretation for hot Jupiter atmospheres may prove challenging: it would
involve anisotropic and dynamic radiative transfer in a coupled
radiation-hydrodynamical flow. On the bright side, it would also imply that the
emergent properties of hot Jupiter atmospheres are more direct tracers of their
atmospheric flows than is the case for Solar System planets.
Radiation-hydrodynamics may also influence radiative transfer in other classes
of hot exoplanetary atmospheres with fast winds.Comment: 25 pages, 4 figures, accepted for publication in ApJ (minor
revisions
A Survey for Circumstellar Disks Around Young Substellar Objects
(Abridged) We have completed the first systematic survey for disks around
spectroscopically identified young brown dwarfs and very low mass stars. We
have obtained L'-band (3.8 um) imaging for 38 very cool objects in IC 348 and
Taurus. Our targets span spectral types from M6 to M9.5 (~100 to ~15 Mjup).
Using the objects' measured spectral types and extinctions, we find that most
of our sample (77%+/-15%) possess intrinsic IR excesses, indicative of disks.
Because the excesses are modest, conventional analyses using only IR colors
would have missed most of the sources with excesses. The observed IR excesses
are correlated with Halpha emission, consistent with a common accretion disk
origin. The excesses can be explained by disk reprocessing of starlight alone;
the implied accretion rates are at least an order of magnitude below typical
values for classical T Tauri stars. The observed distribution of IR excesses
suggests the presence of inner disk holes. The disk frequency appears to be
independent of the mass and age. In the same star-forming regions, disks around
brown dwarfs are at least as long-lived (~3 Myr) as disks around the T Tauri
stars. Altogether, the frequency and properties of young circumstellar disks
appear to be similar from the stellar regime down to the substellar and
planetary-mass regime. This provides prima facie evidence of a common origin
for most stars and brown dwarfs.Comment: ApJ, in press, 28 pages. Minor change to the online, abridged version
of the abstract. No change to the actual pape
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