6,346 research outputs found
Trigonometric Parallaxes for 1,507 Nearby Mid-to-Late M-dwarfs
The MEarth survey is a search for small rocky planets around the smallest,
nearest stars to the Sun as identified by high proper motion with red colors.
We augmented our planetary search time series with lower cadence astrometric
imaging and obtained two million images of approximately 1800 stars suspected
to be mid-to-late M dwarfs. We fit an astrometric model to MEarth's images for
1507 stars and obtained trigonometric distance measurements to each star with
an average precision of 5 milliarcseconds. Our measurements, combined with the
2MASS photometry, allowed us to obtain an absolute K_s magnitude for each star.
In turn, this allows us to better estimate the stellar parameters than those
obtained with photometric estimates alone and to better prioritize the targets
chosen to monitor at high cadence for planetary transits. The MEarth sample is
mostly complete out to a distance of 25 parsecs for stars of type M5.5V and
earlier, and mostly complete for later type stars out to 20 parsecs. We find
eight stars that are within ten parsecs of the Sun for which there did not
exist a published trigonometric parallax distance estimate. We release with
this work a catalog of the trigonometric parallax measurements for 1,507
mid-to-late M-dwarfs, as well as new estimates of their masses and radii.Comment: ApJ, accepted. 36 pages, 8 figures, 2 tables. Please find our data
table here: http://www.cfa.harvard.edu/MEarth/DataDR2.htm
The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood
Rotation is a directly-observable stellar property, and drives magnetic field
generation and activity through a magnetic dynamo. Main sequence stars with
masses below approximately 0.35Msun (mid-to-late M dwarfs) are
fully-convective, and are expected to have a different type of dynamo mechanism
than solar-type stars. Measurements of their rotation rates provide insights
into these mechanisms, but few rotation periods are available for these stars
at field ages. Using photometry from the MEarth transit survey, we measure
rotation periods for 387 nearby, mid-to-late M dwarfs in the Northern
hemisphere, finding periods from 0.1 to 140 days. The typical detected rotator
has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to
1%. We find no period-amplitude relation for stars below 0.25Msun and an
anti-correlation between period and amplitude for higher-mass M dwarfs. We
highlight the existence of older, slowly-rotating stars without H{\alpha}
emission that nevertheless have strong photometric variability. The Galactic
kinematics of our sample is consistent with the local population of G and K
dwarfs, and rotators have metallicities characteristic of the Solar
Neighborhood. We use the W space velocities and established age-velocity
relations to estimate that stars with P<10 days are on average <2 Gyrs, and
that those with P>70 days are about 5 Gyrs. The period distribution is mass
dependent: as the mass decreases, the slowest rotators at a given mass have
longer periods, and the fastest rotators have shorter periods. We find a lack
of stars with intermediate rotation periods. [Abridged]Comment: Accepted to ApJ. Machine readable tables and additional figures are
available in the published article or on reques
A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100 Hour Spitzer Campaign
We present the results of a search for additional bodies in the GJ 1132
system through two methods: photometric transits and transit timing variations
of the known planet. We collected 21 transit observations of GJ 1132b with the
MEarth-South array since 2015. We obtained 100 near-continuous hours of
observations with the Space Telescope, including two transits of GJ
1132b and spanning 60\% of the orbital phase of the maximum period at which
bodies coplanar with GJ 1132b would pass in front of the star. We exclude
transits of additional Mars-sized bodies, such as a second planet or a moon,
with a confidence of 99.7\%. When we combine the mass estimate of the star
(obtained from its parallax and apparent band magnitude) with the stellar
density inferred from our high-cadence light curve (assuming zero
eccentricity), we measure the stellar radius of GJ 1132 to be
, and we refine the radius measurement of
GJ 1132b to . Combined with HARPS RV measurements, we
determine the density of GJ 1132b to be \ g cm, with the
mass determination dominating this uncertainty. We refine the ephemeris of the
system and find no evidence for transit timing variations, which would be
expected if there was a second planet near an orbital resonance with GJ 1132b.Comment: 29 pages, 4 Tables, 8 Figures, Submitted to ApJ. Comments welcom
An expression for the water-sediment moving layer in unsteady flows valid for open channels and embankments
Abstract. During the floods, the effects of sediment transport in river beds are particulary significant and can be studied through the evolution of the water-sediment layer which moves in the lower part of a flow, named "moving layer". Moving layer variations along rivers lead to depositions and erosions and are typically unsteady, but are often tackled with expressions developed for steady (equilibrium) conditions. In this paper, we develop an expression for the moving layer in unsteady conditions and calibrate it with experimental data. During laboratory tests, we have in fact reproduced a rapidly changing unsteady flow by the erosion of a granular steep slope. Results have shown a clear tendency of the moving layer, for fixed discharges, toward equilibrium conditions. Knowing the equilibrium achievement has presented many difficulties, being influenced by the choice of the equilibrium expression and moreover by the estimation of the parameters involved (for example friction angle). Since we used only data relevant to hyper-concentrated mono-dimensional flows for the calibration – occurring for slope gradients in the range 0.03–0.20 – our model can be applied both on open channels and on embankments/dams, providing that the flows can be modelled as mono-dimensional, and that slopes and applied shear stress levels fall within the considered ranges
Renyi generalizations of the conditional quantum mutual information
The conditional quantum mutual information of a tripartite state
is an information quantity which lies at the center of many
problems in quantum information theory. Three of its main properties are that
it is non-negative for any tripartite state, that it decreases under local
operations applied to systems and , and that it obeys the duality
relation for a four-party pure state on systems . The
conditional mutual information also underlies the squashed entanglement, an
entanglement measure that satisfies all of the axioms desired for an
entanglement measure. As such, it has been an open question to find R\'enyi
generalizations of the conditional mutual information, that would allow for a
deeper understanding of the original quantity and find applications beyond the
traditional memoryless setting of quantum information theory. The present paper
addresses this question, by defining different -R\'enyi generalizations
of the conditional mutual information, some of which we can
prove converge to the conditional mutual information in the limit
. Furthermore, we prove that many of these generalizations
satisfy non-negativity, duality, and monotonicity with respect to local
operations on one of the systems or (with it being left as an open
question to prove that monotoniticity holds with respect to local operations on
both systems). The quantities defined here should find applications in quantum
information theory and perhaps even in other areas of physics, but we leave
this for future work. We also state a conjecture regarding the monotonicity of
the R\'enyi conditional mutual informations defined here with respect to the
R\'enyi parameter . We prove that this conjecture is true in some
special cases and when is in a neighborhood of one.Comment: v6: 53 pages, final published versio
Policy Point-Counterpoint: Are the Trump Administration’s Immigration Policies Just or Unjust for Immigrant Children in Particular and the Country in General?
In this policy point-counterpoint, the current administration\u27s immigration policy is examined through the lens of justness
Global Rights, Local Wrongs, and Legal Fixes: An International Human Rights Critique of Immigration and Welfare Reform
The United States enjoys a lofty reputation worldwide as the land of opportunity and dreams, the welcoming home to all who want to be free, the brave new world that embraces huddled masses and offers them limitless possibilities to find freedom, liberty, and happiness. In marked juxtaposition to this welcomeness narrative is the counter-narrative of historic exclusion evidenced by the harsh description of these huddled masses, yearning to breathe free as wretched refuse. Indeed, to describe some immigrants as wretched refuse manifests that Lady Liberty\u27s welcome is, at best, highly selective and, at worst, patently discriminatory. The irony, of course, lies in the basic truth of both narratives - anecdotes imbued with tension throughout
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