Abstract
We measured the mid-infrared (MIR) extinction using Spitzer photometry and spectroscopy (3.6–37
μ
m) for a sample of Milky Way sight lines (mostly) having measured ultraviolet extinction ...curves. We used the pair method to determine the MIR extinction that we then fit with a power law for the continuum and modified Drude profiles for the silicate features. We derived 16 extinction curves having a range of
A
(
V
) (1.8–5.5) and
R
(
V
) values (2.4–4.3). Our sample includes two dense sight lines that have 3
μ
m ice feature detections and weak 2175 Å bumps. The average
A
(
λ
)/
A
(
V
) diffuse sight-line extinction curve we calculate is lower than most previous literature measurements. This agrees better with literature diffuse dust grain models, though it is somewhat higher. The 10
μ
m silicate feature does not correlate with the 2175 Å bump, for the first time providing direct observational confirmation that these two features arise from different grain populations. The strength of the 10
μ
m silicate feature varies by ∼2.5 and is not correlated with
A
(
V
) or
R
(
V
). It is well fit by a modified Drude profile with strong correlations seen between the central wavelength, width, and asymmetry. We do not detect other features with limits in
A
(
λ
)/
A
(
V
) units of 0.0026 (5–10
μ
m), 0.004 (10–20
μ
m), and 0.008 (20–40
μ
m). We find that the standard prescription of estimating
R
(
V
) from
C
×
E
(
K
s
−
V
)/
E
(
B
−
V
) has
C
= −1.14 and a scatter of ∼7%. Using the IRAC 5.6
μ
m band instead of
K
s
gives
C
= −1.03 and the least scatter of ∼3%.
The imaging channel on the Mid-Infrared Instrument (MIRI) is equipped with four coronagraphs that provide high-contrast imaging capabilities for studying faint point sources and extended emission ...that would otherwise be overwhelmed by a bright point-source in its vicinity. Such targets might include stars that are orbited by exoplanets and circumstellar material, mass-loss envelopes around post-main-sequence stars, the near-nuclear environments in active galaxies, and the host galaxies of distant quasars. This paper describes the coronagraphic observing modes of MIRI, as well as performance estimates based on measurements of the MIRI flight model during cryo-vacuum testing. A brief outline of coronagraphic operations is also provided. Finally, simulated MIRI coronagraphic observations of a few astronomical targets are presented for illustration.
Debris Disk Evolution around A Stars Su, K. Y. L; Rieke, G. H; Stansberry, J. A ...
Astrophysical journal/The Astrophysical journal,
12/2006, Letnik:
653, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We report 24 and/or 70 km measurements of 6160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr, based on estimates from the ...literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (63% and 615% at the 1 s level at 24 and 70 km, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 km are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3 s confidence level, the excess rate at 24 and 70 km is 32% and .33% (with an uncertainty of 5%), considerably higher than what has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least 2 orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 km persists longer than that at 24 km. Both results suggest that the debris disk clearing process is more effective in the inner regions.
We present the first scattered-light images of two debris disks around the F8 star HD 104860 and the F0V star HD 192758, respectively ∼45 and ∼67 pc away. We detected these systems in the F110W and ...F160W filters through our reanalysis of archival Hubble Space Telescope (HST) NICMOS data with modern starlight-subtraction techniques. Our image of HD 104860 confirms the morphology previously observed by Herschel in thermal emission with a well-defined ring at a radius of ∼114 au inclined by ∼58°. Although the outer edge profile is consistent with dynamical evolution models, the sharp inner edge suggests sculpting by unseen perturbers. Our images of HD 192758 reveal a disk at radius ∼95 au inclined by ∼59°, never resolved so far. These disks have low scattering albedos of 10% and 13%, respectively, inconsistent with water ice grain compositions. They are reminiscent of several other disks with similar inclination and scattering albedos: Fomalhaut, HD 92945, HD 202628, and HD 207129. They are also very distinct from brighter disks in the same inclination bin, which point to different compositions between these two populations. Varying scattering albedo values can be explained by different grain porosities, chemical compositions, or grain size distributions, which may indicate distinct formation mechanisms or dynamical processes at work in these systems. Finally, these faint disks with large infrared excesses may be representative of an underlying population of systems with low albedo values. Searches with more sensitive instruments on HST or on the James Webb Space Telescope and using state-of-the art starlight-subtraction methods may help discover more of such faint systems.
We analyze a sample of 62600 Spitzer MIPS 24 km sources brighter than 680 kJy and located in the Chandra Deep Field-South to characterize the evolution of the comoving infrared (IR) energy density of ...the universe up to z 6 1. Using published ancillary optical data, we first obtain a nearly complete redshift determination for the 24 km objects associated with R 24 mag counterparts at z 1. These sources represent 655%-60% of the total MIPS 24 km population with f sub(24)k sub(m) 80 kJy, the rest of the sample likely lying at higher redshifts. We then determine an estimate of their total IR luminosities using various libraries of IR spectral energy distributions. We find that the 24 km population at 0.5 z 1 is dominated by "luminous infrared galaxies" (i.e., 10 super(11) L sub( ), L sub(IR) , 10 super(12) L sub( )), the counterparts of which appear to be also luminous at optical wavelengths and tend to be more massive than the majority of optically selected galaxies. A significant number of fainter sources (5 x 10 super(10) L sub( )L sub(IR) , 10 super(11) L sub( ))are also detected at similar distances. We finally derive 15 km and total IR luminosity functions (LFs) up to z 6 1. In agreement with the previous results from the Infrared Space Observatory (ISO) and SCUBA and as expected from the MIPS source number counts, we find very strong evolution of the contribution of the IR-selected population with look-back time. Pure evolution in density is firmly excluded by the data, but we find considerable degeneracy between strict evolution in luminosity and a combination of increases in both density and luminosity L IR a (1 + z) super(3.2) super(+) sub(-) super(0) sub(0) super(.) sub(.) super(7) sub(2) hIR a (1 + z) super(0.7) super(+) sub(-) super(0) sub(0) super(.) sub(.) super(6) sub(2). A significant steepening of the faint-end slope of the IR luminosity function is also unlikely, as it would overproduce the faint 24 km source number counts. Our results imply that the comoving IR energy density of the universe evolves as (1 + z) super(3.9c0.4) up to z 6 1 and that galaxies luminous in the infrared (i.e., L sub(IR) . 10 super(11) L sub( )) are responsible for 70% c 15% of this energy density at z 6 1. Taking into account the contribution of the UV luminosity evolving as (1 + z)6 super(2.5), we infer that these IR-luminous sources dominate the star-forming activity beyond z 6 0.7. The uncertainties affecting these conclusions are largely dominated by the errors in the k-corrections used to convert 24 km fluxes into luminosities.
Context. Characterization of directly imaged exoplanets is one of the most eagerly anticipated science functions of the James Webb Space Telescope. MIRI, the mid-IR instrument, has the capability to ...provide unique spatially resolved photometric data points in a spectral range never before achieved for such objects.
Aims. We aim to present the very first on-sky contrast measurements of the MIRI coronagraphs. In addition to a classical Lyot coronagraph at the longest wavelength, this observing mode implements the concept of the four-quadrant phase mask for the very first time in a space telescope.
Methods. We observed single stars together with a series of reference stars to measure raw contrasts as they are delivered on the detector, as well as reference-subtracted contrasts.
Results. The MIRI coronagraphs achieve raw contrasts better than 10−3 at the smallest angular separations (within 1″) and about 10−5 farther out (beyond 5 ~ 6″). Subtracting the residual diffracted light left behind the coronagraph has the potential to bring the final contrast down to the background- and detector-limited noise floor at most angular separations (a few times 10−5 at less than 1″).
Conclusions. The MIRI coronagraphs behave as expected from simulations. In particular, the raw contrasts for all four coronagraphs are fully consistent with the diffractive model. Contrasts obtained by subtracting reference stars also meet expectations and are fully demonstrated for two four-quadrant phase masks (F1065C and F1140C). The worst contrast, measured at F1550C, is very likely due to a variation in the phase aberrations at the primary mirror during the observations, and not an issue with the coronagraph itself. We did not perform reference star subtraction with the Lyot mask at F2300C, but we anticipate that it would bring the contrast down to the noise floor.
Rationale
Chronic cannabis use is associated with impaired cognitive function. Evidence indicates cannabidiol (CBD) might be beneficial for treating cannabis use disorder. CBD may also have ...pro-cognitive effects; however, its effect on cognition in people with cannabis use disorder is currently unclear.
Objectives
We aimed to assess whether a 4-week CBD treatment impacted cognitive function. We hypothesised that CBD treatment would improve cognition from baseline to week 4, compared to placebo.
Methods
Cognition was assessed as a secondary outcome in a phase 2a randomised, double-blind, parallel-group and placebo-controlled clinical trial of 4-week daily 200 mg, 400 mg and 800 mg CBD for the treatment of cannabis use disorder. Participants had moderate or severe DSM-5 cannabis use disorder and intended to quit cannabis use. Our pre-registered primary cognitive outcome was delayed prose recall. Secondary cognitive outcomes were immediate prose recall, stop signal reaction time, trail-making task performance, verbal fluency and digit span.
Results
Seventy participants were randomly assigned to placebo (
n
= 23), 400 mg CBD (
n
= 24) and 800 mg CBD (
n
= 23). A 200 mg group was eliminated from the trial because it was an inefficacious dose at interim analysis (
n
= 12) and was not analysed here. For the primary cognitive outcome, there was no effect of CBD compared to placebo, evidenced by a lack of dose-by-time interaction at 400 mg (0.46, 95%CIs: − 1.41, 2.54) and 800 mg (0.89, 95%CIs: − 0.99, 2.81). There was no effect of CBD compared to placebo on secondary cognitive outcomes, except backwards digit span which increased following 800 mg CBD (0.30, 95%CIs: 0.02, 0.58).
Conclusions
In this clinical trial for cannabis use disorder, CBD did not influence delayed verbal memory. CBD did not have broad cognitive effects but 800 mg daily treatment may improve working memory manipulation.
Clinical trial registration
The trial was registered with ClinicalTrials.gov (NCT02044809) and the EU Clinical Trials Register (2013–000,361-36).
Decay of Planetary Debris Disks Rieke, G. H; Su, K. Y. L; Stansberry, J. A ...
The Astrophysical journal,
02/2005, Letnik:
620, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We report new Spitzer 24 km photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 km data and 24 and 25 km photometry from the Infrared Space ...Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars with mass close to 2.5 M sub( ), all detected to at least the 67 s level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K - 24 (or 25) km photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission (associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t sub(0)/time dependence, with t sub(0) 6 150 Myr. If anything, stars in binary systems (including Algol-type stars) and l Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10-60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.
We present here analyses of mass-independent effects in the Ti isotope ratios of 17 coarse-grained (compact Type A and Type B) calcium–aluminum-rich inclusions (CAIs) from the Allende CV3 chondrite ...utilizing a method developed for rapid measurement of Ti isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Based on the analyses of the synthetic CMAS glasses doped with varying amounts of Ca, Cr and V, an empirical scheme for correction of potential isobaric interferences and matrix effects was developed. Using this scheme, mass-independent variations of the Ti isotope ratios 46Ti/47Ti, 48Ti/47Ti and 50Ti/47Ti were measured with external reproducibilities (2SD) of ±0.4, ±0.5, and ±1.8 for ε46Ti, ε48Ti, and ε50Ti, respectively, based on repeat measurements of standard glasses. The Ti isotope compositions of the 17 Allende refractory inclusions analyzed here show that most of these CAIs are “normal” with limited variation in their mass-independent Ti isotope composition. One CAI (designated as CMS-1) has a significantly larger mass-independent ε50Ti anomaly and is identified as having Fractionation and Unidentified Nuclear (FUN) effects. The limited range in the Ti isotope compositions of normal CAIs (ε50Ti excesses ranging from 2.9 to 11.4) suggests that they originated from a nebular reservoir that was relatively well-mixed, although not completely homogenized (at the level of precision of the analyses reported here), in its isotopic composition. The distinctive isotopic composition of the FUN CAI CMS-1 indicates that it formed from a reservoir in the protoplanetary disk that was spatially or temporally distinct from that from which the normal CAIs were formed.
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