116 research outputs found
Debris Disks of Members of the Blanco 1 Open Cluster
We have used the Spitzer Space Telescope to obtain Multiband Imaging
Photometer for Spitzer (MIPS) 24 um photometry for 37 members of the ~100 Myr
old open cluster Blanco 1. For the brightest 25 of these stars (where we have
3sigma uncertainties less than 15%), we find significant mid-IR excesses for
eight stars, corresponding to a debris disk detection frequency of about 32%.
The stars with excesses include two A stars, four F dwarfs and two G dwarfs.
The most significant linkage between 24 um excess and any other stellar
property for our Blanco 1 sample of stars is with binarity. Blanco 1 members
that are photometric binaries show few or no detected 24 um excesses whereas a
quarter of the apparently single Blanco 1 members do have excesses. We have
examined the MIPS data for two other clusters of similar age to Blanco 1 -- NGC
2547 and the Pleiades. The AFGK photometric binary star members of both of
these clusters also show a much lower frequency of 24 um excesses compared to
stars that lie near the single-star main sequence. We provide a new
determination of the relation between V-Ks color and Ks-[24] color for main
sequence photospheres based on Hyades members observed with MIPS. As a result
of our analysis of the Hyades data, we identify three low mass Hyades members
as candidates for having debris disks near the MIPS detection limit.Comment: Accepted to Ap
Probing the Gas Content of Late-stage Protoplanetary Disks with N_2H^+
The lifetime of gas in circumstellar disks is a fundamental quantity that informs our understanding of planet formation. Studying disk gas evolution requires measurements of disk masses around stars of various ages. Because H_2 gas is unobservable under most disk conditions, total disk masses are based on indirect tracers such as sub-mm dust and CO emission. The uncertainty in the relation between these tracers and the disk mass increases as the disk evolves. In a few well-studied disks, CO exhibits depletions of up to 100× below the assumed interstellar value. Thus, additional tracers are required to accurately determine the total gas mass. The relative lack of nitrogen found in solid solar system bodies may indicate that it persists in volatile form, making nitrogen-bearing species more robust tracers of gas in more evolved disks. Here we present Atacama Large Millimeter/submillimeter Array detections of N_2H^+ in two mature, ~5–11 Myr old disks in the Upper Scorpius OB Association. Such detections imply the presence of H_2-rich gas and sources of ionization, both required for N_2H^+ formation. The Upper Sco disks also show elevated N_2H^+/CO flux ratios when compared to previously observed disks with ≳10× higher CO fluxes. Based on line ratio predictions from a grid of thermochemical disk models, a significantly reduced CO/H_2 abundance of <10^(−6) for a gas-to-dust ratio of ≳100 is required to produce the observed N_2H^+ fluxes. These systems appear to maintain H_2 gas reservoirs and indicate that carbon- and nitrogen-bearing species follow distinct physical or chemical pathways as disks evolve
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}).Comment: 31 pages, 9 figures, accepted for publication in Ap
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.Comment: article accepted to Ap
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.Comment: 85 pages, 18 figures, 4 tables; accepted for publication in ApJ
Probing the Gas Content of Late-stage Protoplanetary Disks with N_2H^+
The lifetime of gas in circumstellar disks is a fundamental quantity that informs our understanding of planet formation. Studying disk gas evolution requires measurements of disk masses around stars of various ages. Because H_2 gas is unobservable under most disk conditions, total disk masses are based on indirect tracers such as sub-mm dust and CO emission. The uncertainty in the relation between these tracers and the disk mass increases as the disk evolves. In a few well-studied disks, CO exhibits depletions of up to 100× below the assumed interstellar value. Thus, additional tracers are required to accurately determine the total gas mass. The relative lack of nitrogen found in solid solar system bodies may indicate that it persists in volatile form, making nitrogen-bearing species more robust tracers of gas in more evolved disks. Here we present Atacama Large Millimeter/submillimeter Array detections of N_2H^+ in two mature, ~5–11 Myr old disks in the Upper Scorpius OB Association. Such detections imply the presence of H_2-rich gas and sources of ionization, both required for N_2H^+ formation. The Upper Sco disks also show elevated N_2H^+/CO flux ratios when compared to previously observed disks with ≳10× higher CO fluxes. Based on line ratio predictions from a grid of thermochemical disk models, a significantly reduced CO/H_2 abundance of <10^(−6) for a gas-to-dust ratio of ≳100 is required to produce the observed N_2H^+ fluxes. These systems appear to maintain H_2 gas reservoirs and indicate that carbon- and nitrogen-bearing species follow distinct physical or chemical pathways as disks evolve
Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We summarize the progress to date of our Legacy Science Program entitled "The
Formation and Evolution of Planetary Systems" (FEPS) based on observations
obtained with the Spitzer Space Telescope during its first year of operation.
In addition to results obtained from our ground-based preparatory program and
our early validation program, we describe new results from a survey for
near-infrared excess emission from the youngest stars in our sample as well as
a search for cold debris disks around sun-like stars. We discuss the
implications of our findings with respect to current understanding of the
formation and evolution of our own solar system.Comment: 8 postscript pages including 3 figures. To appear in "Spitzer New
Views of the Cosmos" ASP Conference Series, eds. L. Armus et al. FEPS website
at http://feps.as.arizona.ed
Feedback from the heart: emotional learning and memory is controlled by cardiac cycle, interoceptive accuracy and personality
Feedback processing is critical to trial-and-error learning. Here, we examined whether interoceptive signals concerning the state of cardiovascular arousal influence the processing of reinforcing feedback during the learning of ‘emotional’ face-name pairs, with subsequent effects on retrieval. Participants (N = 29) engaged in a learning task of face-name pairs (fearful, neutral, happy faces). Correct and incorrect learning decisions were reinforced by auditory feedback, which was delivered either at cardiac systole (on the heartbeat, when baroreceptors signal the contraction of the heart to the brain), or at diastole (between heartbeats during baroreceptor quiescence). We discovered a cardiac influence on feedback processing that enhanced the learning of fearful faces in people with heightened interoceptive ability. Individuals with enhanced accuracy on a heartbeat counting task learned fearful face-name pairs better when feedback was given at systole than at diastole. This effect was not present for neutral and happy faces. At retrieval, we also observed related effects of personality: First, individuals scoring higher for extraversion showed poorer retrieval accuracy. These individuals additionally manifested lower resting heart rate and lower state anxiety, suggesting that attenuated levels of cardiovascular arousal in extraverts underlies poorer performance. Second, higher extraversion scores predicted higher emotional intensity ratings of fearful faces reinforced at systole. Third, individuals scoring higher for neuroticism showed higher retrieval confidence for fearful faces reinforced at diastole. Our results show that cardiac signals shape feedback processing to influence learning of fearful faces, an effect underpinned by personality differences linked to psychophysiological arousal
Spitzer Observations of G Dwarfs in the Pleiades: Circumstellar Debris Disks at 100 Myr Age
Fluxes and upper limits in the wavelength range from 3.6 to 70 microns from
the Spitzer Space Telescope are provided for twenty solar-mass Pleiades
members. One of these stars shows a probable mid-IR excess and two others have
possible excesses, presumably due to circumstellar debris disks. For the star
with the largest, most secure excess flux at MIPS wavelengths, HII1101, we
derive Log(L[dust]/L[Sun]) ~ -3.8 and an estimated debris disk mass of 4.2 x
10^-5 M(Earth) for an assumed uniform dust grain size of 10 microns If the
stars with detected excesses are interpreted as stars with relatively recent,
large collision events producing a transient excess of small dust particles,
the frequency of such disk transients is about ~ 10 % for our ~ 100 Myr,
Pleiades G dwarf sample. For the stars without detected 24-70 micron excesses,
the upper limits to their fluxes correspond to approximate 3 sigma upper limits
to their disk masses of 6 x 10^-6 M(Earth) using the MIPS 24 micron upper
limit, or 2 x 10^-4 M(Earth) using the MIPS 70 micron limit. These upper limit
disk masses (for "warm" and "cold" dust, respectively) are roughly consistent,
but somewhat lower than, predictions of a heuristic model for the evolution of
an "average" solar-mass star's debris disk based on extrapolation backwards in
time from current properties of the Sun's Kuiper belt.Comment: 32 postscript pages including 8 figues and 3 tables. To appear in the
Astronomical Journa
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