60 research outputs found
PREDOMINANTLY LOW METALLICITIES MEASURED IN A STRATIFIED SAMPLE OF LYMAN LIMIT SYSTEMS AT Z = 3.7
We measured metallicities for 33 z = 3.4–4.2 absorption line systems drawn from a sample of H i-selected-Lyman limit systems (LLSs) identified in Sloan Digital Sky Survey (SDSS) quasar spectra and stratified based on metal line features. We obtained higher-resolution spectra with the Keck Echellette Spectrograph and Imager, selecting targets according to our stratification scheme in an effort to fully sample the LLS population metallicity distribution. We established a plausible range of H i column densities and measured column densities (or limits) for ions of carbon, silicon, and aluminum, finding ionization-corrected metallicities or upper limits. Interestingly, our ionization models were better constrained with enhanced α-to-aluminum abundances, with a median abundance ratio of [α/Al] = 0.3. Measured metallicities were generally low, ranging from [M/H] = −3 to −1.68, with even lower metallicities likely for some systems with upper limits. Using survival statistics to incorporate limits, we constructed the cumulative distribution function (CDF) for LLS metallicities. Recent models of galaxy evolution propose that galaxies replenish their gas from the low-metallicity intergalactic medium (IGM) via high-density H i "flows" and eject enriched interstellar gas via outflows. Thus, there has been some expectation that LLSs at the peak of cosmic star formation (z ≈ 3) might have a bimodal metallicity distribution. We modeled our CDF as a mix of two Gaussian distributions, one reflecting the metallicity of the IGM and the other representative of the interstellar medium of star-forming galaxies. This bimodal distribution yielded a poor fit. A single Gaussian distribution better represented the sample with a low mean metallicity of [M/H] ≈ −2.5.Massachusetts Institute of Technology. Undergraduate Research Opportunities ProgramNational Science Foundation (U.S.) (Award AST-1109915
WASP-4b Arrived Early for the TESS Mission
The Transiting Exoplanet Survey Satellite (TESS) recently observed 18
transits of the hot Jupiter WASP-4b. The sequence of transits occurred 81.6
11.7 seconds earlier than had been predicted, based on data stretching
back to 2007. This is unlikely to be the result of a clock error, because TESS
observations of other hot Jupiters (WASP-6b, 18b, and 46b) are compatible with
a constant period, ruling out an 81.6-second offset at the 6.4 level.
The 1.3-day orbital period of WASP-4b appears to be decreasing at a rate of
milliseconds per year. The apparent period change
might be caused by tidal orbital decay or apsidal precession, although both
interpretations have shortcomings. The gravitational influence of a third body
is another possibility, though at present there is minimal evidence for such a
body. Further observations are needed to confirm and understand the timing
variation.Comment: AJ accepte
TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844
Data from the newly-commissioned \textit{Transiting Exoplanet Survey
Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf
located 15 pc away. The planet has a radius of and
orbits the star every 11 hours. Although the existence of an atmosphere around
such a strongly irradiated planet is questionable, the star is bright enough
(, ) for this possibility to be investigated with transit and
occultation spectroscopy. The star's brightness and the planet's short period
will also facilitate the measurement of the planet's mass through Doppler
spectroscopy.Comment: 10 pages, 4 figures. Submitted to ApJ Letters. This letter makes use
of the TESS Alert data, which is currently in a beta test phase, using data
from the pipelines at the TESS Science Office and at the TESS Science
Processing Operations Cente
TESS Discovery of a Transiting Super-Earth in the Mensae System
We report the detection of a transiting planet around Mensae (HD
39091), using data from the Transiting Exoplanet Survey Satellite (TESS). The
solar-type host star is unusually bright (V=5.7) and was already known to host
a Jovian planet on a highly eccentric, 5.7-year orbit. The newly discovered
planet has a size of and an orbital period of 6.27
days. Radial-velocity data from the HARPS and AAT/UCLES archives also displays
a 6.27-day periodicity, confirming the existence of the planet and leading to a
mass determination of . The star's proximity and
brightness will facilitate further investigations, such as atmospheric
spectroscopy, asteroseismology, the Rossiter--McLaughlin effect, astrometry,
and direct imaging.Comment: Accepted for publication ApJ Letters. This letter makes use of the
TESS Alert data, which is currently in a beta test phase. The discovery light
curve is included in a table inside the arxiv submissio
NuSTAR resolves the first dual AGN above 10 keV in SWIFT J2028.5+2543
We have discovered heavy obscuration in the dual active galactic nucleus (AGN) in the Swift/Burst Alert Telescope (BAT) source SWIFT J2028.5+2543 using Nuclear Spectroscopic Telescope Array (NuSTAR). While an early XMM-Newton study suggested the emission was mainly from NGC 6921, the superior spatial resolution of NuSTAR above 10 keV resolves the Swift/BAT emission into two sources associated with the nearby galaxies MCG +04-48-002 and NGC 6921 (z = 0.014) with a projected separation of 25.3 kpc (91"). NuSTAR's sensitivity above 10 keV finds both are heavily obscured to Compton-thick levels (NH ~ (1–2) × 1024 cm-2) and contribute equally to the BAT detection (Lint10-50 keV ~ 6 × 1042 erg s-1). The observed luminosity of both sources is severely diminished in the 2–10 keV band (Lobs2-10 keV < 0.1 x Lint2-10 keV ), illustrating the importance of >10 keV surveys like those with NuSTAR and Swift/BAT. Compared to archival X-ray data, MCG +04-48-002 shows significant variability (>3) between observations. Despite being bright X-ray AGNs, they are difficult to detect using optical emission-line diagnostics because MCG +04-48-002 is identified as a starburst/composite because of the high rates of star formation from a luminous infrared galaxy while NGC 6921 is only classified as a LINER using line detection limits. SWIFT J2028.5+2543 is the first dual AGN resolved above 10 keV and is the second most heavily obscured dual AGN discovered to date in the X-rays other than NGC 6240
TESS full orbital phase curve of the WASP-18b system
We present a visible-light full orbital phase curve of the transiting planet
WASP-18b measured by the TESS Mission. The phase curve includes the transit,
secondary eclipse, and sinusoidal modulations across the orbital phase shaped
by the planet's atmospheric characteristics and the star-planet gravitational
interaction. We measure the beaming (Doppler boosting) and tidal ellipsoidal
distortion phase modulations and show that the amplitudes of both agree with
theoretical expectations. We find that the light from the planet's day-side
hemisphere occulted during secondary eclipse, with a relative brightness of
ppm, is dominated by thermal emission, leading to an upper
limit on the geometric albedo in the TESS band of 0.048 (2). We also
detect the phase modulation due to the planet's atmosphere longitudinal
brightness distribution. We find that its maximum is well-aligned with the
sub-stellar point, to within 2.9 deg (2). We do not detect light from
the planet's night-side hemisphere, with an upper limit of 43 ppm (2),
which is 13 % of the day-side brightness. The low albedo, lack of atmospheric
phase shift, and inefficient heat distribution from the day to night
hemispheres that we deduce from our analysis are consistent with theoretical
expectations and similar findings for other strongly irradiated gas giant
planets. This work demonstrates the potential of TESS data for studying full
orbital phase curves of transiting systems. Finally, we complement our study by
looking for transit timing variations (TTVs) in the TESS data and combined with
previously published transit times, although we do not find a statistically
significant TTV signal.Comment: V2: Added another TESS Sector of data to the analysis, added TTV
analysis, accepted to A
A Model for Type 2 Coronal Line Forest (CLiF) AGNs
We present a model for the classification of Coronal Line Forest Active Galactic Nuclei (CLiF AGNs). CLiF
AGNs are of special interest due to their remarkably large number of emission lines, especially forbidden highionization
lines (FHILs). Rose et al. suggest that their emission is dominated by reflection from the inner wall of
the obscuring region rather than direct emission from the accretion disk. This makes CLiF AGNs laboratories to
test AGN-torus models. Modeling an AGN as an accreting supermassive black hole surrounded by a cylinder of
dust and gas, we show a relationship between the viewing angle and the revealed area of the inner wall. From the
revealed area, we can determine the amount of FHIL emission at various angles. We calculate the strength of
[Fe VII]λ6087 emission for a number of intermediate angles (30°, 40°, and 50°) and compare the results with the
luminosity of the observed emission line from six known CLiF AGNs. We find that there is good agreement
between our model and the observational results. The model also enables us to determine the relationship between
the type 2:type 1 AGN fraction vs the ratio of torus height to radius, h/r
- …