5,917 research outputs found
Observational constraints on dust disk lifetimes : implications for planet formation
Thus far our impressions regarding the evolutionary time scales for young circumstellar disks
have been based on small number statistics. Over the past decade, however, in addition to preci-
sion study of individual star/disk systems, substantial observational effort has been invested in
obtaining less detailed data on large numbers of objects in young star clusters. This has resulted
in a plethora of information now enabling statistical studies of disk evolutionary diagnostics.
Along an ordinate one can measure disk presence or strength through indicators such as ul-
traviolet/blue excess or spectroscopic emission lines tracing accretion, infrared excess tracing
dust, or millimeter flux measuring mass. Along an abscissa one can track stellar age. While
bulk trends in disk indicators versus age are evident, observational errors affecting both axes,
combined with systematic errors in our understanding of stellar ages, both cloud and bias any
such trends. Thus detailed understanding of the physical processes involved in disk dissipation
and of the relevant time scales remains elusive. Nevertheless, a clear effect in current data that
is unlikely to be altered by data analysis improvements is the dispersion in disk lifetimes. Inner
accretion disks are traced by near-infrared emission. Moderating a generally declining trend in
near-infared continuum excess and excess frequency with age over <1 to 8±4 Myr, is the fact
that a substantial fraction of rather young (<1 Myr old) stars apparently have already lost their
inner accretion disks while a significant number of rather old (8-16 Myr) stars apparently still
retain inner accretion disks. The age at which evidence for inner accretion disks ceases to be
apparent for the vast majority (~90%) of stars is in the range 3-8 Myr. More distant, terrestrial
zone dust is traced by mid-infrared emission where sufficient sensitivity and uniform data collec-
tion are only now being realized with data return from the Spitzer Space Telescope. Constraints
on mid-disk dissipation and disk clearing trends with radius are forthcoming
Near-Infrared Spectroscopy of 2M0441+2301 AabBab: A Quadruple System Spanning the Stellar to Planetary Mass Regimes
We present Keck/NIRC2 and OSIRIS near-infrared imaging and spectroscopy of
2M0441+2301 AabBab, a young (1--3 Myr) hierarchical quadruple system comprising
a low-mass star, two brown dwarfs, and a planetary-mass companion in Taurus.
All four components show spectroscopic signs of low surface gravity, and both
2M0441+2301 Aa and Ab possess Pa emission indicating they each harbor
accretion subdisks. Astrometry spanning 2008--2014 reveals orbital motion in
both the Aab (0.23" separation) and Bab (0.095" separation) pairs, although the
implied orbital periods of 300 years means dynamical masses will not be
possible in the near future. The faintest component (2M0441+2301 Bb) has an
angular -band shape, strong molecular absorption (VO, CO, HO, and FeH),
and shallow alkali lines, confirming its young age, late spectral type (L1
1), and low temperature (1800~K). With individual masses of
200 Mjup, 35 5 Mjup, 19 3 Mjup, and 9.8 1.8
Mjup, 2M0441+2301 AabBab is the lowest-mass quadruple system known. Its
hierarchical orbital architecture and mass ratios imply that it formed from the
collapse and fragmentation of a molecular cloud core, demonstrating that
planetary-mass companions can originate from a stellar-like pathway analogous
to higher-mass quadruple star systems as first speculated by Todorov et al.
More generally, cloud fragmentation may be an important formation pathway for
the massive exoplanets that are now regularly being imaged on wide orbits.Comment: Added an additional mention of an already cited reference, upon
indirect reques
Debris Disk Radiative Transfer Simulation Tool (DDS)
A WWW interface for the simulation of spectral energy distributions of
optically thin dust configurations with an embedded radiative source is
presented. The density distribution, radiative source, and dust parameters can
be selected either from an internal database or defined by the user. This tool
is optimized for studying circumstellar debris disks where large grains are
expected to determine the far-infrared through millimeter dust reemission
spectral energy distribution. The tool is available at
http://aida28.mpia-hd.mpg.de/~swolf/ddsComment: Comp. Phys. Comm, 2005, in pres
Precision photometric monitoring of very low mass Sigma Orionis cluster members: variability and rotation at a few Myr
We present high-precision photometry on 107 variable low-mass stars and brown
dwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-band
photometric monitoring within two fields, encompassing 153 confirmed or
candidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot.
We are sensitive to brightness changes on time scales from 10 minutes to two
weeks with amplitudes as low as 0.004 magnitudes, and find variability on these
time scales in nearly 70% of cluster members. We identify both periodic and
aperiodic modes of variability, as well as semi-periodic rapid fading events
that are not accounted for by the standard explanations of rotational
modulation of surface features or accretion. We have incorporated both optical
and infrared color data to uncover trends in variability with mass and
circumstellar disks. While the data confirm that the lowest-mass objects
(M<0.2M_\odot) rotate more rapidly than the 0.2-0.5 M_\odot members, they do
not support a direct connection between rotation rate and the presence of a
disk. Finally, we speculate on the origin of irregular variability in cluster
members with no evidence for disks or accretion.Comment: 40 pages, 18 figures, accepted for publication in ApJS. Link to
electronic figures correcte
Determinants of residential water demand in Germany
In this paper we econometrically analyze the impact of several economic, environmental and social determinants for the average per capita demand for water and sewage in about 600 water supply areas in Germany. Besides prices, income and household size, we also consider the effects of population age, the share of wells, and rainfall and temperature during the summer months on water demand. We also attempt to explain regional differences in per capita residential water consumption, which is currently about 30 % lower in the new federal states than in the old states. Our estimate for the price elasticity of -0.229 suggests that the response of residential water demand in Germany is rather inelastic, but no significant difference could be found between both regions. In contrast, the income elasticity in the new states is found to be 0.685 which is more than double that of the old states. Differences in prices and income alone explain the largest part of the current gap in residential water use between the two regions. Our results further suggest that household size, the share of wells and summer rainfall have a negative impact on water demand. In contrast, higher age appears to be associated with higher water use. We also find (weak) evidence for an impact of rainfall but not of temperature on residential water use. Our findings imply that future research should include analyses of household- level data to further explore the effects of socio-economic determinants, and analyses of panel data to adequately study the effects of climate change on residential water use. --
The Stellar Populations of Praesepe and Coma Berenices
We present the results of a stellar membership survey of the nearby open
clusters Praesepe and Coma Berenices. We have combined archival survey data
from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compile proper motions
and photometry for ~5 million sources over 300 deg^2. Of these sources, 1010
stars in Praesepe and 98 stars in Coma Ber are identified as candidate members
with probability >80%; 442 and 61 are identified as high-probability candidates
for the first time. We estimate that this survey is >90% complete across a wide
range of spectral types (F0 to M5 in Praesepe, F5 to M6 in Coma Ber). We have
also investigated the stellar mass dependence of each cluster's mass and radius
in order to quantify the role of mass segregation and tidal stripping in
shaping the present-day mass function and spatial distribution of stars.
Praesepe shows clear evidence of mass segregation across the full stellar mass
range; Coma Ber does not show any clear trend, but low number statistics would
mask a trend of the same magnitude as in Praesepe. The mass function for
Praesepe (t~600 Myr; M~500 Msun) follows a power law consistent with that of
the field present-day mass function, suggesting that any mass-dependent tidal
stripping could have removed only the lowest-mass members (<0.15 Msun). Coma
Ber, which is younger but much less massive (t~400 Myr; M~100 Msun), follows a
significantly shallower power law. This suggests that some tidal stripping has
occurred, but the low-mass stellar population has not been strongly depleted
down to the survey completeness limit (~0.12 Msun).Comment: Accepted to AJ; 14 pages, 10 figures, 5 tables + 2 online-only table
Empirical Isochrones for Low Mass Stars in Nearby Young Associations
Absolute ages of young stars are important for many issues in pre-main
sequence stellar and circumstellar evolution but are long recognized as
difficult to derive and calibrate. In this paper, we use literature spectral
types and photometry to construct empirical isochrones in HR diagrams for
low-mass stars and brown dwarfs in the eta Cha, epsilon Cha, and TW Hya
Associations and the beta Pic and Tuc-Hor Moving Groups. A successful theory of
pre-main sequence evolution should match the shapes of the stellar loci for
these groups of young stars. However, when comparing the combined empirical
isochrones to isochrones predicted from evolutionary models, discrepancies lead
to a spectral type (mass) dependence in stellar age estimates. Improved
prescriptions for convection and boundary conditions in the latest models of
pre-main sequence models lead to a significantly improved correspondence
between empirical and model isochrones, with small offsets at low temperatures
that may be explained by observational uncertainties or by model limitations.
Independent of model predictions, linear fits to combined stellar loci of these
regions provide a simple empirical method to order clusters by luminosity with
a reduced dependence on spectral type. Age estimates calculated from various
sets of modern models that reproduce Li depletion boundary ages of the beta Pic
Moving Group also imply a ~4 Myr age for the low mass members of the Upper Sco
OB Association, which is younger than the 11 Myr age that has been recently
estimated for intermediate mass members.Comment: Accepted by ApJ, 18 page
Age-Related Observations of Low Mass Pre-Main and Young Main Sequence Stars (Invited Review)
This overview summarizes the age dating methods available for young sub-solar
mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell
(HR) diagram, spectroscopic surface gravity indicators, and lithium depletion;
asteroseismology is also showing recent promise. Near and beyond the zero-age
main sequence, rotation period or vsini and activity (coronal and
chromospheric) diagnostics along with lithium depletion serve as age proxies.
Other authors in this volume present more detail in each of the aforementioned
areas. Herein, I focus on pre-main sequence HR diagrams and address the
questions: Do empirical young cluster isochrones match theoretical isochrones?
Do isochrones predict stellar ages consistent with those derived via other
independent techniques? Do the observed apparent luminosity spreads at constant
effective temperature correspond to true age spreads? While definitive answers
to these questions are not provided, some methods of progression are outlined.Comment: to appear in IAU Symposium 258, "Ages of Stars", E.E. Mamajek, D.R.
Soderblom, R.F.G. Wyse, ed
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