16,391 research outputs found
Ion thermal effects in oscillating multi-ion plasma sheath theory
The effects of ion temperature are discussed in a two-ion electron plasma and
for a model applicable to the oscillating sheath theory that has recently been
much in the focus of researchers. The differences between the fluid and kinetic
models have been pointed out, as well as the differences between the
approximative kinetic description (which involves the expansion of the plasma
dispersion function), and the exact kinetic description. It is shown that the
approximative kinetic description, first, can not describe the additional
acoustic mode which naturally exists in the plasma with an additional ion
population with a finite temperature, and, second, it yields an inaccurate
Landau damping of the bulk ion acoustic mode. The reasons for these two
failures are described. In addition to this, a fluid model is presented that is
capable of capturing both of these features that are missing in the
approximative kinetic description, i.e., two (fast and slow) ion acoustic
modes, and the corresponding Landau damping of both modes
On the L¹ norm of an exponential sum involving the divisor function
In this paper, we obtain bounds on the L¹-norm of the sum ∑_(n≤x)τ(n)e(αn) where τ(n) is the divisor function
Pre-main-sequence population in NGC 1893 region: X-ray properties
Continuing the attempt to understand the properties of the stellar content in
the young cluster NGC 1893 we have carried out a comprehensive multi-wavelength
study of the region. The present study focuses on the X-ray properties of
T-Tauri Stars (TTSs) in the NGC 1893 region. We found a correlation between the
X-ray luminosity, , and the stellar mass (in the range 0.22.0 \msun) of
TTSs in the NGC 1893 region, similar to those reported in some other young
clusters, however the value of the power-law slope obtained in the present
study ( 0.9) for NGC 1893 is smaller than those (1.4 - 3.6)
reported in the case of TMC, ONC, IC 348 and Chameleon star forming regions.
However, the slope in the case of Class III sources (Weak line TTSs) is found
to be comparable to that reported in the case of NGC 6611 ( 1.1). It is
found that the presence of circumstellar disks has no influence on the X-ray
emission. The X-ray luminosity for both CTTSs and WTTSs is found to decrease
systematically with age (in the range 0.4 Myr - 5 Myr). The decrease of
the X-ray luminosity of TTSs (slope -0.6) in the case of NGC 1893 seems
to be faster than observed in the case of other star-forming regions (slope
-0.2 to -0.5). There is indication that the sources having relatively large NIR
excess have relatively lower values. TTSs in NGC 1893 do not follow the
well established X-ray activity - rotation relation as in the case of
main-sequence stars.Comment: 10 pages, 7 figures, Accepted for publication in New Astronom
Cooperative Emission of a Coherent Superflash of Light
We investigate the transient coherent transmission of light through an
optically thick cold stron-tium gas. We observe a coherent superflash just
after an abrupt probe extinction, with peak intensity more than three times the
incident one. We show that this coherent superflash is a direct signature of
the cooperative forward emission of the atoms. By engineering fast transient
phenomena on the incident field, we give a clear and simple picture of the
physical mechanisms at play.Comment: 4 Fig., 5 page
The molecular complex associated with the Galactic HII region Sh2-90: a possible site of triggered star formation
We investigate the star formation activity in the molecular complex
associated with the Galactic HII region Sh2-90, using radio-continuum maps
obtained at 1280 MHz and 610 MHz, Herschel Hi-GAL observations at 70 -- 500
microns, and deep near-infrared observation at JHK bands, along with Spitzer
observations. Sh2-90 presents a bubble morphology in the mid-IR (size ~ 0.9 pc
x 1.6 pc). Radio observations suggest it is an evolved HII region with an
electron density ~ 144 cm^-3, emission measure ~ 6.7 x 10^4 cm^-6 pc and a
ionized mass ~ 55 Msun. From Hi-GAL observations it is found that the HII
region is part of an elongated extended molecular cloud (size ~ 5.6 pc x 9.7
pc, H_2 column density >= 3 x 10^21 cm^-2 and dust temperature 18 -- 27 K) of
total mass >= 1 x 10^4 Msun. We identify the ionizing cluster of Sh2-90, the
main exciting star being an O8--O9 V star. Five cold dust clumps (mass ~ 8 --
95 Msun), four mid-IR blobs around B stars, and a compact HII region are found
at the edge of the bubble.The velocity information derived from CO (J=3-2) data
cubes suggests that most of them are associated with the Sh2-90 region. 129
YSOs are identified (Class I, Class II, and near-IR excess sources). The
majority of the YSOs are low mass (<= 3 Msun) sources and they are distributed
mostly in the regions of high column density. Four candidate Class 0/I MYSOs
have been found; they will possibly evolve to stars of mass >= 15 Msun. We
suggest multi-generation star formation is present in the complex. From the
evidences of interaction, the time scales involved and the evolutionary status
of stellar/protostellar sources, we argue that the star formation at the
immediate border/edges of Sh2-90 might have been triggered by the expanding HII
region. However, several young sources in this complex are probably formed by
some other processes.Comment: 22 pages, 22 figures, accepted for publication in Astronomy and
Astrophysic
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