19,571 research outputs found
Kernel conditional quantile estimation via reduction revisited
Quantile regression refers to the process of estimating the quantiles of a conditional distribution and has many important applications within econometrics and data mining, among other domains. In this paper, we show how to estimate these conditional quantile functions within a Bayes risk minimization framework using a Gaussian process prior. The resulting non-parametric probabilistic model is easy to implement and allows non-crossing quantile functions to be enforced. Moreover, it can directly be used in combination with tools and extensions of standard Gaussian Processes such as principled hyperparameter estimation, sparsification, and quantile regression with input-dependent noise rates. No existing approach enjoys all of these desirable properties. Experiments on benchmark datasets show that our method is competitive with state-of-the-art approaches.
Brown dwarfs in the Hyades and beyond?
We have used both the Low-Resolution Imaging Spectrograph and the HIRES
echelle spectrograph on the Keck telescopes to obtain spectra of twelve
candidate members of the Hyades cluster identified by Leggett and Hawkins
(1988, 1989). All of the objects are chromospherically-active, late-type
M-dwarfs, with H equivalent widths varying from 1 to 30\AA. Based on
our measured radial velocities, the level of stellar activity and other
spectroscopic features, only one of the twelve stars has properties consistent
with cluster membership. We consider how this result affects estimates of the
luminosity and mass function of the Hyades cluster. Five of the eleven field
stars have weak K I 7665/7699\AA and CaH absorption as compared with M-dwarf
standards of the same spectral type, suggesting a lower surface gravity. Two of
these sources, LH0416+14 and LH0419+15, exhibit significant lithium 6708 \AA
absorption. Based partly on parallax measurements by the US Naval Observatory
(Harris et al, 1998), we identify all five as likely to be young, pre-main
sequence objects in or near the Taurus-Auriga association at distances of
between 150 and 250 parsecs. A comparison with theoretical models of pre-main
sequence stars indicates masses of less than 0.05 M.Comment: to appear in AJ, January 1999; 34 pages, (Latex format), including 10
embedded postscript figures and two table
The distance to a star forming region in the Outer arm of the Galaxy
We performed astrometric observations with the VLBA of WB89-437, an H2O maser
source in the Outer spiral arm of the Galaxy. We measure an annual parallax of
0.167 +/- 0.006 mas, corresponding to a heliocentric distance of 6.0 +/- 0.2
kpc or a Galactocentric distance of 13.4 +/- 0.2 kpc. This value for the
heliocentric distance is considerably smaller than the kinematic distance of
8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135
degrees. We also measured the full space motion of the object and find a large
peculiar motion of ~20 km/s toward the Galactic center. This peculiar motion
explains the large error in the kinematic distance estimate. We also find that
WB89-437 has the same rotation speed as the LSR, providing more evidence for a
flat rotation curve and thus the presence of dark matter in the outer Galaxy.Comment: The Astrophysical Journal, accepted, 16 pages, 4 Figure
5-micron photometry of late-type dwarfs
We present narrowband-M photometry of nine low-mass dwarfs with spectral
types ranging from M2.5 to L0.5. Combining the (L'-M') colours derived from our
observations with data from the literature, we find colours consistent with a
Rayleigh-Jeans flux distribution for spectral types earlier than M5, but
enhanced F_3.8/F_4.7 flux ratios (negative (L'-M') colours) at later spectral
types. This probably reflects increased absorption at M' due to the CO
fundamental band. We compare our results against recent model predictions and
briefly discuss the implications.Comment: accepted for the Astronomical Journa
An empirical analysis of smart contracts: platforms, applications, and design patterns
Smart contracts are computer programs that can be consistently executed by a
network of mutually distrusting nodes, without the arbitration of a trusted
authority. Because of their resilience to tampering, smart contracts are
appealing in many scenarios, especially in those which require transfers of
money to respect certain agreed rules (like in financial services and in
games). Over the last few years many platforms for smart contracts have been
proposed, and some of them have been actually implemented and used. We study
how the notion of smart contract is interpreted in some of these platforms.
Focussing on the two most widespread ones, Bitcoin and Ethereum, we quantify
the usage of smart contracts in relation to their application domain. We also
analyse the most common programming patterns in Ethereum, where the source code
of smart contracts is available.Comment: WTSC 201
Multi-Wavelength Study of Sgr A*: The Short Time Scale Variability
To understand the correlation and the radiation mechanism of flare emission
in different wavelength bands, we have coordinated a number of telescopes to
observe SgrA* simultaneously. We focus only on one aspect of the preliminary
results of our multi-wavelength observing campaigns, namely, the short time
scale variability of emission from SgrA* in near-IR, X-ray and radio
wavelengths. The structure function analysis indicate most of the power
spectral density is detected on hourly time scales in all wavelength bands. We
also report minute time scale variability at 7 and 13mm placing a strong
constraint on the nature of the variable emission. The hourly time scale
variability can be explained in the context of a model in which the peak
frequency of emission shifts toward lower frequencies as a self-absorbed
synchrotron source expands adiabatically near the acceleration site. The short
time scale variability, on the other hand, places a strong constraint on the
size of the emitting region. Assuming that rapid minute time scale fluctuations
of the emission is optically thick in radio wavelength, light travel arguments
requires relativistic particle energy, thus suggesting the presence of outflow
from SgrA*.Comment: 9 pages, 4 figures, The Galactic Center: A Window on the Nuclear
Environment of Disk Galaxies ASP Conference Series, 2010 eds: M. Morris, D.
Q. Wang and F. Yua
Constraining the History of the Sagittarius Dwarf Galaxy Using Observations of its Tidal Debris
We present a comparison of semi-analytic models of the phase-space structure
of tidal debris with observations of stars associated with the Sagittarius
dwarf galaxy (Sgr). We find that many features in the data can be explained by
these models. The properties of stars 10-15 degrees away from the center of Sgr
--- in particular, the orientation of material perpendicular to Sgr's orbit
(c.f. Alard 1996) and the kink in the velocity gradient (Ibata et al 1997) ---
are consistent with those expected for unbound material stripped during the
most recent pericentric passage ~50 Myrs ago. The break in the slope of the
surface density seen by Mateo, Olszewski & Morrison (1998) at ~ b=-35 can be
understood as marking the end of this material. However, the detections beyond
this point are unlikely to represent debris in a trailing streamer, torn from
Sgr during the immediately preceding passage ~0.7 Gyrs ago, but are more
plausibly explained by a leading streamer of material that was lost more that 1
Gyr ago and has wrapped all the way around the Galaxy. The observations
reported in Majewski et al (1999) also support this hypothesis. We determine
debris models with these properties on orbits that are consistent with the
currently known positions and velocities of Sgr in Galactic potentials with
halo components that have circular velocities v_circ=140-200 km/s. The best
match to the data is obtained in models where Sgr currently has a mass of ~10^9
M_sun and has orbited the Galaxy for at least the last 1 Gyr, during which time
it has reduced its mass by a factor of 2-3, or luminosity by an amount
equivalent to ~10% of the total luminosity of the Galactic halo. These numbers
suggest that Sgr is rapidly disrupting and unlikely to survive beyond a few
more pericentric passages.Comment: 19 pages, 5 figures, accepted to Astronomical Journa
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