240 research outputs found
Accretion in Brown Dwarfs: an Infrared View
This paper presents a study of the accretion properties of 19 very low mass
objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8
objects we obtained high resolution Halpha profiles and determined mass
accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in
emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using
objects for which we have both a determination of Lacc from Halpha and a Pabeta
detection, we show that the correlation between the Pabeta luminosity and
luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus,
extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure
Lacc also in the substellar regime. The results were less conclusive for
Brgamma, which was detected only in 2 objects, neither of which had an Halpha
estimate of Macc. Using the relation between L(Pab) and Lacc we determined the
accretion rate for all the objects in our sample (including those with no
Halpha spectrum), more than doubling the number of substellar objects with
known Macc. When plotted as a function of the mass of the central object
together with data from the literature, our results confirm the trend of lower
Macc for lower Mstar, although with a large spread. Some of the spread is
probably due to an age effect; our very young objects in rho-Oph have on
average an accretion rate at least one order of magnitude higher than objects
of similar mass in older regions. As a side product, we found that the width of
Halpha measured at 10% peak intensity is not only a qualitative indicator of
accretion, but can be used to obtain a quantitative estimate of Macc over a
large mass range, from T Tauri stars to brown dwarfs. Finally, we found that
some of our objects show evidence of mass-loss.Comment: 11 pages, 8 figures, A&A in pres
Measuring Fundamental Parameters of Substellar Objects. II: Masses and Radii
We present mass and radius derivations for a sample of very young, mid- to
late M, low-mass stellar and substellar objects in Upper Sco and Taurus. In a
previous paper, we determined effective temperatures and surface gravities for
these targets, from an analysis of their high-resolution optical spectra and
comparisons to the latest synthetic spectra. We now derive extinctions, radii,
masses and luminosities by combining our previous results with observed
photometry, surface fluxes from the synthetic spectra and the known cluster
distances. These are the first mass and radius estimates for young, very low
mass bodies that are independent of theoretical evolutionary models (though our
estimates do depend on spectral modeling). We find that for most of our sample,
our derived mass-radius and mass-luminosity relationships are in very good
agreement with the theoretical predictions. However, our results diverge from
the evolutionary model values for the coolest, lowest-mass targets: our
inferred radii and luminosities are significantly larger than predicted for
these objects at the likely cluster ages, causing them to appear much younger
than expected. We suggest that uncertainties in the evolutionary models - e.g.,
in the choice of initial conditions and/or treatment of interior convection -
may be responsible for this discrepancy. Finally, two of our late-M objects
(USco 128 and 130) appear to have masses close to the deuterium-fusion boundary
(9--14 Jupiters, within a factor of 2). This conclusion is primarily a
consequence of their considerable faintness compared to other targets with
similar extinction, spectral type and temperature (difference of 1 mag). Our
result suggests that the faintest young late-M or cooler objects may be
significantly lower in mass than the current theoretical tracks indicate.Comment: 54 pages, incl. 5 figs, accepted Ap
Development of an Economical, Linear CCD Based Spectrometer
Spectrometry provides vital information about the composition of electromagnetic radiation by measuring wavelengths of light over a wide range of the electromagnetic spectrum. In this work, design and construction of an economical linear CCD spectrometer and a computer application is presented. A sample of the light source travelling through an optical fiber enters the device and the spectrum produced by a diffraction grating is projected on a linear CCD detector. The data is sampled using a 10- bit ADC to the microcontroller memory and finally transferred to the computer via a full speed USB connection. In the computer, the data is digitally processed and displayed through a graphical user interface with several useful functionalities. The device is sensitive to electromagnetic waves of wavelengths ranging from 400 to 800 nm, which is decided by the sensitivity of the silicon CCD detector. The device performs in par with commercially available handheld spectrometers and is equipped with wavelength and amplitude calibration capabilities.Keywords: spectrometer, instrumentation, CC
Environmental Repercussions of Gem Mining in Sri Lanka
As an industry, gem mining has both negative and positive impacts on economic and social perspectives. But this has generated a severe negative impact on the environment and health perspectives. Most of these impacts cannot be recovered for an extended period of time while causing the deaths of all living organisms on Earth. This study has portrayed the environmental repercussions of traditional and mechanical gem mining techniques practiced by the gem miners in Ratnapura while discussing the remedies that can be adapted to overcome those environmental impacts with the objective of identifying the environmental repercussion of gem industry in Ratnapura district. The population of this study consisted of the gem mining lands in Ratnapura and out of that selected the five of tunnel gem mine land, backhoe gem mine land and river gem mines land as the sample of this study. Collection of the data was done through the observation and discussions conducted with the gem miners to identify the environmental repercussions of gem mining activities. The analysis of the data was done as a narrative analysis of qualitative research. As a major gem mining country Sri Lanka practiced three techniques for gem mining as tunnel gem mining, backhoe gem mining and river gem mining. Although there is a huge contribution to the economic development of the country, these techniques have several impacts on the environment and many other sources. As an environmental impact there are the contamination of water, erosion of soil, deforestation, losing the nutrients of the soil, loss of animal habitats, loss of biodiversity and many more. To avoid these impacts miners can adapt environmentally friendly methods for gem mining and it is necessary to regulate the mining activities by the government to reduce the impact of mining activities.
© 2022 The Authors. Published by Department of Estate Management and Valuation, University of Sri Jayewardenepura
Keywords: Environmental repercussions; Gem mining techniques; Remedial measures; Gemmin
New sub-millimeter limits on dust in the 55 Cancri planetary system
We present new, high-sensitivity sub-millimeter observations towards 55
Cancri, a nearby G8 star with one, or possibly two, known planetary
companion(s). Our 850 m map, obtained with the SCUBA instrument on the
James Clerk Maxwell Telescope, shows three peaks of emission at the 2.5 mJy
level in the vicinity of the star's position. However, the observed peaks are
25\arcsec--40\arcsec away from the star and a deep -band optical image
reveals faint point sources that coincide with two of the sub-millimeter peaks.
Thus, we do not find evidence for dust emission spatially associated with 55
Cancri. The excess 60 m emission detected with ISO may originate from one
or more of the 850 m peaks that we attribute to background sources. Our
new results, together with the HST/NICMOS coronographic images in the
near-infrared, place stringent limits on the amount of dust in this planetary
system, and argue against the existence of a detectable circumstellar dust disk
around 55 Cnc.Comment: 11 pages, 2 PostScript figures, to appear in The Astrophysical
Journal Letter
Light enhancement in surface-enhanced raman scattering at oblique incidence
Surface enhanced Raman scattering (SERS) measurements have been carried out at different focusing conditions using objective lenses of different numerical apertures. The experimentally observed dependence of SERS intensity of thiophenol-coated Ag nano-islands shows a close-to-linear scaling with the collection aperture. The linear relationship breaks down for large numerical apertures, which suggests that the scattering is anisotropic. Numerical simulations of realistically shaped Ag nano-islands were carried out, and the spatial distribution of hot-spots has been revealed at different heights near the nano-islands. Local field enhancements of up to 100 times were estimated. The simulation also suggests an explanation for the anisotropy in the scattering observed for larger numerical aperture objectives. This appears to be due to a reduction in the local field enhancement as the electric field vector component in the plane of the shallow metal islands reduces at larger angles of incidence
Input-to-state stability of infinite-dimensional control systems
We develop tools for investigation of input-to-state stability (ISS) of
infinite-dimensional control systems. We show that for certain classes of
admissible inputs the existence of an ISS-Lyapunov function implies the
input-to-state stability of a system. Then for the case of systems described by
abstract equations in Banach spaces we develop two methods of construction of
local and global ISS-Lyapunov functions. We prove a linearization principle
that allows a construction of a local ISS-Lyapunov function for a system which
linear approximation is ISS. In order to study interconnections of nonlinear
infinite-dimensional systems, we generalize the small-gain theorem to the case
of infinite-dimensional systems and provide a way to construct an ISS-Lyapunov
function for an entire interconnection, if ISS-Lyapunov functions for
subsystems are known and the small-gain condition is satisfied. We illustrate
the theory on examples of linear and semilinear reaction-diffusion equations.Comment: 33 page
A disk census for the nearest group of young stars: Mid-infrared observations of the TW Hydrae Association
A group of young, active stars in the vicinity of TW Hydrae has recently been
identified as a possible physical association with a common origin. Given its
proximity (50 pc), age (10 Myr) and abundance of binary systems,
the TW Hya Association is ideally suited to studies of diversity and evolution
of circumstellar disks. Here we present mid-infrared observations of 15
candidate members of the group, 11 of which have no previous flux measurements
at wavelengths longer than 2m. We report the discovery of a possible
10m excess in CD -337795, which may be due to a circumstellar
disk or a faint, as yet undetected binary companion. Of the other stars, only
TW Hya, HD 98800, Hen 3-600A, and HR 4796A -- all of which were detected by
IRAS -- show excess thermal emission. Our 10m flux measurements for the
remaining members of the Association are consistent with photospheric emission,
allowing us to rule out dusty inner disks. In light of these findings, we
discuss the origin and age of the TW Hya Association as well as implications
for disk evolution timescales.Comment: 10 pages and 1 PostScript figure, accepted for publication in The
Astrophysical Journal Letter
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