84 research outputs found
Study of Plasma-Based Acceleration for High Energy Physics and Other Applications
In the framework of this thesis, laser pulses, electron- and proton-beams are investigated as driver for plasma-based electron acceleration. In particular, two possible mid-term applications are studied: High energy lepton collider and injectors for synchrotron light sources
The Coevality of Young Binary Systems
Multiple star systems are commonly assumed to form coevally; they thus
provide the anchor for most calibrations of stellar evolutionary models. In
this paper we study the binary population of the Taurus-Auriga association,
using the component positions in an HR diagram in order to quantify the
frequency and degree of coevality in young binary systems. After identifying
and rejecting the systems that are known to be affected by systematic errors
(due to further multiplicity or obscuration by circumstellar material), we find
that the relative binary ages, |Delta log(tau)|, have an overall dispersion of
sigma~0.40 dex. Random pairs of Taurus members are coeval only to within
sigma~0.58 dex, indicating that Taurus binaries are indeed more coeval than the
association as a whole. However, the distribution of |Delta log(tau)| suggests
two populations, with ~2/3 of the sample appearing coeval to within the errors
(sigma~0.16 dex) and the other ~1/3 distributed in an extended tail reaching
|Delta log(tau)|~0.4-0.9 dex. To explain the finding of a multi-peaked
distribution, we suggest that the tail of the differential age distribution
includes unrecognized hierarchical multiples, stars seen in scattered light, or
stars with disk contamination; additional followup is required to rule out or
correct for these explanations. The relative coevality of binary systems does
not depend significantly on the system mass, mass ratio, or separation. Indeed,
any pair of Taurus members wider than ~10' (~0.7 pc) shows the full age spread
of the association.Comment: 23 pages, 11 figures; accepted to Ap
Unusually Wide Binaries: Are They Wide or Unusual?
We describe an astrometric and spectroscopic campaign to confirm the youth
and association of a complete sample of candidate wide companions in Taurus and
Upper Sco. Our survey found fifteen new binary systems (3 in Taurus and 12 in
Upper Sco) with separations of 3-30" (500-5000 AU) among all of the known
members with masses of 2.5-0.012 Msun. The total sample of 49 wide systems in
these two regions conforms to only some expectations from field multiplicity
surveys. Higher-mass stars have a higher frequency of wide binary companions,
and there is a marked paucity of wide binary systems near the substellar
regime. However, the separation distribution appears to be log-flat, rather
than declining as in the field, and the mass ratio distribution is more biased
toward similar-mass companions than the IMF or the field G dwarf distribution.
The maximum separation also shows no evidence of a limit at <5000 AU until the
abrupt cessation of any wide binary formation at system masses of ~0.3 Msun. We
attribute this result to the post-natal dynamical sculpting that occurs for
most field systems; our binary systems will escape to the field intact, but
most field stars are formed in denser clusters and do not. In summary, only
wide binary systems with total masses <0.3 Msun appear to be "unusually wide".Comment: Accepted to ApJ; 23 pages, 9 figures, 9 tables in emulateapj. Table 6
is online-only and is attached as tab6.te
Evidence for a palaeo-subglacial lake on the Antarctic continental shelf
Subglacial lakes are widespread beneath the Antarctic Ice Sheet but their control on ice-sheet dynamics and their ability to harbour life remain poorly characterized. Here we present evidence for a palaeo-subglacial lake on the Antarctic continental shelf. A distinct sediment facies recovered from a bedrock basin in Pine Island Bay indicates deposition within a low-energy lake environment. Diffusive-advection modelling demonstrates that low chloride concentrations in the pore water of the corresponding sediments can only be explained by initial deposition of this facies in a freshwater setting. These observations indicate that an active subglacial meltwater network, similar to that observed beneath the extant ice sheet, was also active during the last glacial period. It also provides a new framework for refining the exploration of these unique environments
TrES-2: The First Transiting Planet in the Kepler Field
We announce the discovery of the second transiting hot Jupiter discovered by
the Trans-atlantic Exoplanet Survey. The planet, which we dub TrES-2, orbits
the nearby star GSC 03549-02811 every 2.47063 days. From high-resolution
spectra, we determine that the star has T_eff = 5960 +/- 100 K and log(g) = 4.4
+/- 0.2, implying a spectral type of G0V and a mass of 1.08 +0.11/-0.05 M_sun.
High-precision radial-velocity measurements confirm a sinusoidal variation with
the period and phase predicted by the photometry, and rule out the presence of
line-bisector variations that would indicate that the spectroscopic orbit is
spurious. We estimate a planetary mass of 1.28 +0.09/-0.04 M_Jup. We model B,
r, R, and I photometric timeseries of the 1.4%-deep transits and find a
planetary radius of 1.24 +0.09/-0.06 R_Jup. This planet lies within the field
of view of the NASA Kepler mission, ensuring that hundreds of upcoming transits
will be monitored with exquisite precision and permitting a host of
unprecedented investigations.Comment: Accepted for publication in ApJL. 15 pages, 2 figure
A Chandra Observation of the Obscured Star-Forming Complex W40
The young stellar cluster illuminating the W40 H II region, one of the
nearest massive star forming regions, has been observed with the ACIS detector
on board the Chandra X-ray Observatory. Due to its high obscuration, this is a
poorly-studied stellar cluster with only a handful of bright stars visible in
the optical band, including three OB stars identified as primary excitation
sources. We detect 225 X-ray sources, of which 85% are confidently identified
as young stellar members of the region. Two potential distances of the cluster,
260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity
function to be universal, it supports a 600 pc distance as a lower limit for
W40 and a total population of at least 600 stars down to 0.1 Mo under the
assumption of a coeval population with a uniform obscuration. In fact, there is
strong spatial variation in Ks-band-excess disk fraction and non-uniform
obscuration due to a dust lane that is identified in absorption in optical,
infrared and X-ray. The dust lane is likely part of a ring of material which
includes the molecular core within W40. In contrast to the likely ongoing star
formation in the dust lane, the molecular core is inactive. The star cluster
has a spherical morphology, an isothermal sphere density profile, and mass
segregation down to 1.5 Mo. However, other cluster properties, including a
\leq{1} Myr age estimate and ongoing star formation, indicate that the cluster
is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a
young stellar object are also reported.Comment: Accepted for publication in The Astrophysical Journal. 60 pages, 16
figure
Mapping the Shores of the Brown Dwarf Desert II: Multiple Star Formation in Taurus-Auriga
We have conducted a high-resolution imaging study of the Taurus-Auriga
star-forming region in order to characterize the primordial outcome of multiple
star formation and the extent of the brown dwarf desert. Our survey identified
16 new binary companions to primary stars with masses of 0.25-2.5 Msun, raising
the total number of binary pairs (including components of high-order multiples)
with separations of 3--5000 AU to 90. We find that ~2/3--3/4 of all Taurus
members are multiple systems of two or more stars, while the other ~1/4--1/3
appear to have formed as single stars; the distribution of high-order
multiplicity suggests that fragmentation into a wide binary has no impact on
the subsequent probability that either component will fragment again. The
separation distribution for solar-type stars (0.7--2.5 Msun) is nearly log-flat
over separations of 3--5000 AU, but lower-mass stars (0.25--0.7 Msun) show a
paucity of binary companions with separations of >200 AU. Across this full mass
range, companion masses are well described with a linear-flat function; all
system mass ratios (q=M_B/M_A) are equally probable, apparently including
substellar companions. Our results are broadly consistent with the two expected
modes of binary formation (freefall fragmentation on large scales and disk
fragmentation on small scales), but the distributions provide some clues as to
the epochs at which the companions are likely to form.Comment: Accepted to ApJ; 32 pages, 7 figures, 6 tables in emulateapj forma
First evidence for a late LGM subglacial lake in Pine Island Bay, Antarctica
Subglacial lakes are widespread beneath the Antarctic Ice Sheet and as a source for subglacial meltwater they are assumed to modulate ice stream velocity. Further, the evacuation of subglacial meltwater at the ice sheet margin influences ocean circulation and geochemical cycles. However, despite their importanceâ subglacial lakes are one of the least explored environments on our planet. As a consequence, their importance for ice sheet dynamics and their ability to harbour life remain poorly characterised.
We present the first direct evidence for a palaeo-subglacial lake on the Antarctic continental shelf, document- ing that subglacial meltwater was stored during the last glacial period and evacuated during the subsequent deglaciation. A distinct sediment facies observed in a core recovered from a small bedrock basin in Pine Island Bay, Amundsen Sea, is indicative of deposition within a low-energy subglacial lake setting. Diffusive modelling demonstrates that low chloride concentrations in the pore water of this characteristic sediment facies can only be explained by original deposition in a freshwater setting. We also show that the location of the subglacial lake within a basin on the inner shelf is consistent with the predicted distribution of subglacial lakes based on bathymetric data. This finding will enable future modelling studies to investigate how the geometry and capacity of subglacial lake systems can influence ice dynamics when the substrate and profile of the ice sheet is known â especially in the highly sensitive area known as the "weak underbelly" of the WAIS.
With the exception of a direct lake water access at Subglacial Lake Vostok, and some centimetres of sediment retrieval from Subglacial Lake Whillans, the subglacial hydrological system in Antarctica has hitherto mostly been explored using remote sensing and numerical models that suggest the number of potential lake sites to more than 12.000.
Our study not only provides first empirical evidence for a palaeo-subglacial lake but also delivers a framework for investigating and refining exploration of these unique subglacial lake environments and their sediments beneath thick contemporary ice sheets. Our approach, however, is easier and cheaper to conduct by using ship borne coring equipment on the seasonal ice-free continental shelf
What are the temperatures of T Tauri stars? - Constraints from coeval formation of young eclipsing binaries
We show how the assumption of coeval formation can be used to constrain the
effective temperatures of the components of young eclipsing double-lined
spectroscopic binaries. Our method extends the approach of White et al. (1999)
to a two-step analysis. The first step compares evolutionary models to the
observed masses and radii and selects those models that predict ages that are
consistent with coeval formation. The second step then uses these models to
constrain the effective temperatures. We applied the method on literature
values of the stellar parameters of the eclipsing binaries RX J0529.4+0041 A
and V1174 Ori and confirm that V1174 Ori A has dwarf-like temperatures at an
age of 9 Myrs, while we cannot draw any conclusions for RX J0529.4+0041 A and
V1174 Ori B. Considering these binaries, we find that none of the evolutionary
models gives coeval solutions simultaneously in mass, radius and effective
temperature.Comment: accepted for publication in A&A, sect. 7 on 24/06/2005; 7 pages + 2
appendices, 5 figures, 5 table
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