1,362 research outputs found
HO distribution in the disc of HD 100546 and HD 163296: the role of dust dynamics and planet--disc interaction
[Abridged] Far-infrared observations with Herschel revealed a surprisingly
low abundance of cold-water reservoirs in protoplanetary discs. On the other
hand, a handful of discs show emission of hot water transitions excited at
temperatures above a few hundred Kelvin. In particular, the protoplanetary
discs around the Herbig Ae stars HD 100546 and HD 163296 show opposite trends
in terms of cold versus hot water emission: in the first case, the ground-state
transitions are detected and the high-J lines are undetected, while the trend
is opposite in HD 163296. We performed a spectral analysis using the
thermo-chemical model DALI. We find that HD 163296 is characterised by a
water-rich (abundance ) hot inner disc (within the snowline)
and a water-poor () outer disc: the relative abundance may be due
to the thermal desorption of icy grains that have migrated inward. Remarkably,
the size of the HO emitting region corresponds to a narrow dust gap visible
in the millimeter continuum at au with ALMA. The low-J lines detected
in HD 100546 instead imply an abundance of a few in the cold outer
disc ( au). The emitting region of the cold HO transitions is
spatially coincident with that of the HO ice previously seen in the
near-infrared. Notably, millimetre observations with ALMA reveal the presence
of a large dust gap between nearly 40 and 150 au, likely opened by a massive
embedded protoplanet. In both discs, we find that the warm molecular layer in
the outer region (beyond the snow line) is highly depleted of water molecules,
implying an oxygen-poor chemical composition of the gas. We speculate that
gas-phase oxygen in the outer disc is readily depleted and its distribution in
the disc is tightly coupled to the dynamics of the dust grains.Comment: Accepted for publication on A&
Black-Hole Spin Dependence in the Light Curves of Tidal Disruption Events
A star orbiting a supermassive black hole can be tidally disrupted if the
black hole's gravitational tidal field exceeds the star's self gravity at
pericenter. Some of this stellar tidal debris can become gravitationally bound
to the black hole, leading to a bright electromagnetic flare with bolometric
luminosity proportional to the rate at which material falls back to pericenter.
In the Newtonian limit, this flare will have a light curve that scales as
t^-5/3 if the tidal debris has a flat distribution in binding energy. We
investigate the time dependence of the black-hole mass accretion rate when
tidal disruption occurs close enough the black hole that relativistic effects
are significant. We find that for orbits with pericenters comparable to the
radius of the marginally bound circular orbit, relativistic effects can double
the peak accretion rate and halve the time it takes to reach this peak
accretion rate. The accretion rate depends on both the magnitude of the
black-hole spin and its orientation with respect to the stellar orbit; for
orbits with a given pericenter radius in Boyer-Lindquist coordinates, a maximal
black-hole spin anti-aligned with the orbital angular momentum leads to the
largest peak accretion rate.Comment: 16 pages, 15 figures, 1 table, PRD published versio
The GAPS Programme with HARPS-N at TNG VIII: Observations of the Rossiter-McLaughlin effect and characterisation of the transiting planetary systems HAT-P-36 and WASP-11/HAT-P-10
We determine the true and the projected obliquity of HAT-P-36 and
WASP-11/HAT-P-10 systems, respectively, which are both composed of a relatively
cool star and a hot-Jupiter planet. Thanks to the high-resolution spectrograph
HARPS-N, we observed the Rossiter-McLaughlin effect for both the systems by
acquiring precise radial-velocity measurements during planetary transit events.
We also present photometric observations comprising six light curves covering
five transit events, obtained using three medium-class telescopes and the
telescope-defocussing technique. One transit of WASP-11/HAT-P-10 was followed
simultaneously from two observatories. The three transit light curves of
HAT-P-36b show anomalies that are attributable to starspot complexes on the
surface of the parent star, in agreement with the analysis of its spectra that
indicate a moderate activity. By analysing the complete HATNet data set of
HAT-P-36, we estimated the stellar rotation period by detecting a periodic
photometric modulation in the light curve caused by star spots, obtaining
Prot=15.3 days, which implies that the inclination of the stellar rotational
axis with respect to the line of sight is 65 degree. We used the new
spectroscopic and photometric data to revise the main physical parameters and
measure the sky-projected misalignment angle of the two systems. We found
\lambda=-14 degree for HAT-P-36 and \lambda=7 degree for WASP-11/HAT-P-10,
indicating in both cases a good spin-orbit alignment. In the case of HAT-P-36,
we also measured its real obliquity, which turned out to be 25 degrees.Comment: 18 pages, 14 figure
The GAPS Programme with HARPS-N@TNG XIV. Investigating giant planet migration history via improved eccentricity and mass determination for 231 transiting planets
We carried out a Bayesian homogeneous determination of the orbital parameters
of 231 transiting giant planets (TGPs) that are alone or have distant
companions; we employed DE-MCMC methods to analyse radial-velocity (RV) data
from the literature and 782 new high-accuracy RVs obtained with the HARPS-N
spectrograph for 45 systems over 3 years. Our work yields the largest sample of
systems with a transiting giant exoplanet and coherently determined orbital,
planetary, and stellar parameters. We found that the orbital parameters of TGPs
in non-compact planetary systems are clearly shaped by tides raised by their
host stars. Indeed, the most eccentric planets have relatively large orbital
separations and/or high mass ratios, as expected from the equilibrium tide
theory. This feature would be the outcome of high-eccentricity migration (HEM).
The distribution of , where and are the semi-major axis
and the Roche limit, for well-determined circular orbits peaks at 2.5; this
also agrees with expectations from the HEM. The few planets of our sample with
circular orbits and values may have migrated through disc-planet
interactions instead of HEM. By comparing circularisation times with stellar
ages, we found that hot Jupiters with au have modified tidal quality
factors are
required to explain the presence of eccentric planets at the same orbital
distance. As a by-product of our analysis, we detected a non-zero eccentricity
for HAT-P-29; we determined that five planets that were previously regarded to
have hints of non-zero eccentricity have circular orbits or undetermined
eccentricities; we unveiled curvatures caused by distant companions in the RV
time series of HAT-P-2, HAT-P-22, and HAT-P-29; and we revised the planetary
parameters of CoRoT-1b.Comment: 44 pages (16 pages of main text and figures), 11 figures, 5
longtables, published in Astronomy and Astrophysics, Volume 602, A107 (2017).
Tables with new HARPS-N and TRES radial-velocity data (Tables 1 and 2),
stellar parameters (Table 7), orbital parameters and RV jitter (Table 8), and
planet physical parameters (Table 9) are available as ancillary files
(sidebar on the right
The GAPS Programme with HARPS-N@TNG VI: The Curious Case of TrES-4b
We revisit the TrES-4 system parameters based on high-precision HARPS-N
radial-velocity measurements and new photometric light curves. A combined
spectroscopic and photometric analysis allows us to determine a spectroscopic
orbit with an amplitude m s. The derived mass of TrES-4b is
found to be , significantly lower than
previously reported. Combined with the large radius () inferred from our analysis, TrES-4b becomes
the second-lowest density transiting hot Jupiter known. We discuss several
scenarios to explain the puzzling discrepancy in the mass of TrES-4b in the
context of the exotic class of highly inflated transiting giant planets.Comment: 5 pages, 4 figures, Letter accepted for publication in Astronomy and
Astrophysic
The GAPS programme with HARPS-N@TNG IV: A planetary system around XO-2S
We performed an intensive radial velocity monitoring of XO-2S, the wide
companion of the transiting planet-host XO-2N, using HARPS-N at TNG in the
framework of the GAPS programme. The radial velocity measurements indicate the
presence of a new planetary system formed by a planet that is slightly more
massive than Jupiter at 0.48 au and a Saturn-mass planet at 0.13 au. Both
planetary orbits are moderately eccentric and were found to be dynamically
stable. There are also indications of a long-term trend in the radial
velocities. This is the first confirmed case of a wide binary whose components
both host planets, one of which is transiting, which makes the XO-2 system a
unique laboratory for understanding the diversity of planetary systems.Comment: 7 pages, 3 figures, accepted on A&A Lette
Testing external photoevaporation in the -Orionis cluster with spectroscopy and disk mass measurements
The evolution of protoplanetary disks is regulated by an interplay of several
processes, either internal to the system or related to the environment. As most
of the stars and planets have formed in massive stellar clusters, studying the
effects of UV radiation on disk evolution is of paramount importance. Here we
test the impact of external photoevaporation on the evolution of disks in the
Orionis cluster by conducting the first combined large-scale UV to IR
spectroscopic and mm-continuum survey of this region. We study a sample of 50
targets located at increasing distances from the central, OB system
Ori. We combine new VLT/X-Shooter spectra with new and previously published
ALMA measurements of disk dust and gas fluxes and masses. We confirm the
previously found decrease of in the inner 0.5 pc of the
cluster. This is particularly evident when considering the disks around the
more massive stars ( 0.4 ), where those located in the inner
part ( 0.5 pc) have about an order of magnitude lower than
the more distant ones. About half of the sample is located in the region of the
vs expected by models of external
photoevaporation, namely showing shorter disk lifetimes. These are observed for
all targets with projected separation from Ori 0.5 pc, proving
that the presence of a massive stellar system affects disk evolution. External
photoevaporation is a viable mechanism to explain the observed shorter disk
lifetimes and lower in the inner 0.5 pc of the cluster.
Follow-up observations of the low stellar mass targets are crucial to confirm
the dependence of the external photoevaporation process with stellar host mass.
This work confirms that the effects of external photoevaporation are
significant down to impinging radiation as low as G.Comment: Accepted for publication on Astronomy & Astrophysics. 13 pages, 7
figures + appendix. Abstract abridged to meet arXiv requirement
Direct Formation of Supermassive Black Holes via Multi-Scale Gas Inflows in Galaxy Mergers
Observations of distant bright quasars suggest that billion solar mass
supermassive black holes (SMBHs) were already in place less than a billion
years after the Big Bang. Models in which light black hole seeds form by the
collapse of primordial metal-free stars cannot explain their rapid appearance
due to inefficient gas accretion. Alternatively, these black holes may form by
direct collapse of gas at the center of protogalaxies. However, this requires
metal-free gas that does not cool efficiently and thus is not turned into
stars, in contrast with the rapid metal enrichment of protogalaxies. Here we
use a numerical simulation to show that mergers between massive protogalaxies
naturally produce the required central gas accumulation with no need to
suppress star formation. Merger-driven gas inflows produce an unstable, massive
nuclear gas disk. Within the disk a second gas inflow accumulates more than 100
million solar masses of gas in a sub-parsec scale cloud in one hundred thousand
years. The cloud undergoes gravitational collapse, which eventually leads to
the formation of a massive black hole. The black hole can grow to a billion
solar masses in less than a billion years by accreting gas from the surrounding
disk.Comment: 26 pages, 4 Figures, submitted to Nature (includes Supplementary
Information
New insights on the AU-scale circumstellar structure of FU Orionis
We report new near-infrared, long-baseline interferometric observations at
the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and
VLTI interferometers. This young stellar object has been observed on 42 nights
over a period of 6 years from 1998 to 2003. We have obtained 287 independent
measurements of the fringe visibility with 6 different baselines ranging from
20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane
coverage, coupled with the published spectral energy distribution data, allows
us to test the accretion disk scenario. We find that the most probable
explanation for these observations is that FU Ori hosts an active accretion
disk whose temperature law is consistent with standard models. We are able to
constrain the geometry of the disk, including an inclination of 55 deg and a
position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is
detected in the data (at the two-sigma level) from the longest baselines, which
we interpret as a possible disk hot-spot or companion. However, the oscillation
in our best data set is best explained with an unresolved spot located at a
projected distance of 10 AU at the 130 deg position angle and with a magnitude
difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at
a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a
companion of the central FU Ori system on an extremely eccentric orbit. We
speculate that the close encounter of this putative companion and the central
star could be the explanation of the initial photometric rise of the luminosity
of this object
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