1,953 research outputs found
A "diamond-ring" star: the unusual morphologic structure of a young (multiple?) object
We have observed IRAS06468-0325 obtaining optical and infrared images through
IJHKs and L' filters, K-band low-resolution spectroscopy, together with
millimetre line observations of CO and CS. IRAS06468-0325 has a very unusual
and enigmatic morphology with two components: a bright, close to point-like
source (the diamond) and a sharp-edge ring-like structure (the ring). The
source is not detected in the optical, at wavelengths shorter than the I-band.
The diamond is seen in all the imaging bands observed. The ring-like structure
in IRAS06468-0325 is clearly seen in the I, J, H, and Ks. It is not detected in
the L'-band image. Infrared colours of the diamond are compatible with excess
circumstellar emission and a young stellar nature. A strongly non-gaussian and
moderately bright CO(1-0) and {13}CO(2-1) lines are seen towards
IRAS06468-0325, at v_{LSR} of 30.5 km s{-1} (corresponding to a kinematic
distance of 3 kpc). Very weak C{18}O(2-1) and CS(2-1) lines were detected.
K-band spectra of the diamond and of the ring are similar both in the slope of
the continuum and in the presence of lines supporting the idea that the ring is
reflected light from the diamond. With the current data, a few different
scenarios are possible to explain the morphology of this object. However, the
available data seem to favour that the morphology of IRAS06468-0325 correspond
to a young stellar multiple system in a transient stage where a binary
co-exists with a circumbinary disc, similar to the case of GG Tau. In this
case, the sharpness of the well-defined ring may be due to tidal truncation
from dynamic interactions between components in a binary or multiple stellar
system. IRAS06468-0325 may be an important rare case that illustrates a
short-lived stage of the process of binary or multiple star formation.Comment: 7 pages, 6 figure
Implications of the LHC two-photon signal for two-Higgs-doublet models
We study the implications for Two Higgs Doublet Models of the recent
announcement at the LHC giving a tantalizing hint for a Higgs boson of mass 125
GeV decaying into two photons. We require that the experimental result be
within a factor of two of the theoretical Standard Model prediction, and
analyze the type I and type II models as well as the lepton-specific and
flipped models, subject to this requirement. It is assumed that there is no new
physics other than two Higgs doublets. In all of the models, we display the
allowed region of parameter space taking the recent LHC announcement at face
value, and we analyze the , , and
expectations in these allowed regions. Throughout the entire range of parameter
space allowed by the constraint, the number of events for Higgs
decays into , and are not changed from the Standard Model
by more than a factor of two. In contrast, in the Lepton Specific model, decays
to are very sensitive across the entire -allowed region.Comment: Latex, 6 pages, 4 figures; v2 - added 2 reference
Mass-degenerate Higgs bosons at 125 GeV in the Two-Higgs-Doublet Model
The analysis of the Higgs boson data by the ATLAS and CMS Collaborations
appears to exhibit an excess of h --> gamma\gamma events above the Standard
Model (SM) expectations; whereas no significant excess is observed in h --> ZZ*
--> {four lepton} events, albeit with large statistical uncertainty due to the
small data sample. These results (assuming they persist with further data)
could be explained by a pair of nearly mass-degenerate scalars, one of which is
a SM-like Higgs boson and the other is a scalar with suppressed couplings to
W+W- and ZZ. In the two Higgs doublet model, the observed \gamma\gamma and ZZ*
--> {four lepton} data can be reproduced by an approximately degenerate CP-even
(h) and CP-odd (A) Higgs boson for values of \sin(\beta-\alpha) near unity and
0.7 < \tan\beta < 1. An enhanced \gamma\gamma signal can also arise in cases
where m_h ~ m_H, m_H ~ m_A, or m_h ~ m_H ~ m_A. Since the ZZ* --> {four lepton}
signal derives primarily from a SM-like Higgs boson whereas the \gamma\gamma
signal receives contributions from two (or more) nearly mass-degenerate states,
one would expect a slightly different invariant mass peak in the ZZ* --> {four
lepton} and \gamma\gamma channels. The phenomenological consequences of such
models can be tested with additional Higgs data that will be collected at the
LHC in the near future.Comment: 18 pages, 19 pdf figures, v2: references added, v3&v4: added refs and
explanation
Identification issues in models for underreported counts
In this note we study the conditions under which leading models for underreported counts are identified. In particular, we highlight a peculiar identification problem that afflicts two of the most popular models in this class
On the use of robust regression in econometrics
The use of robust regression estimators has gained popularity among applied econometricians. The main argument invoked to justify the use of the robust estimators is that they provide efficiency gains in the presence of outliers or non-normal errors. Unfortunately, most practitioners seem to be unaware of the fact that heteroskedastic and skewed errors can dramatically affect the properties of these estimators. In this paper we reconsider the interpretation of the specific robust estimator that has become popular in applied econometrics, and conclude that its use in this context cannot be generally recommended
Specification and Testing of Models Estimated by Quadrature
This paper proposes a test to check the specification of models with unobserved individual effects integrated out by quadrature and also a simple way of increasing the flexibility of this type of model. The results of a Monte Carlo study and an application using a well-known data set illustrate the finite sample properties of the proposed methods and their implementation in practice
Quantile regression with clustered data
We show that the quantile regression estimator is consistent and asymptotically normal when the error terms are correlated within clusters but independent across clusters. A consistent estimator of the covariance matrix of the asymptotic distribution is provided and we propose a specification test capable of detecting the presence of intra-cluster correlation. A small simulation study
illustrates the finite sample performance of the test and of the covariance matrix estimator
Quantiles for Fractions and Other Mixed Data
This paper studies the estimation of quantile regression for fractional data, focusing on the case where there are mass-points at zero or/and one. More generally, we propose a simple strategy for the estimation of the conditional quantiles of data from mixed distributions, which combines standard results on the estimation of censored and Box-Cox quantile regressions. The implementation of the proposed method is illustrated using a well-known dataset
A cautionary note on tests for overidentifying restrictions
Tests of overidentifying restrictions are widely used in practice. However, there is often confusion about the nature of their null hypothesis and about the interpretation of their outcome. In this note we argue that these tests give little information on whether the instruments are correlated with the errors of the underlaying economic model and on whether they identify parameters of interest
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