528 research outputs found
Thomas James (ca. 1593-ca. 1635)
... James emerged from the shadows for only one brief period. In 1631, he was selected by some Bristol merchants to see if there was a passage leading from Hudson's newly discovered bay into the fabled Pacific. After an unsuccessful search (for there was no passage), James wintered near the northeast corner of Charlton Island, and returned to Bristol the following summer. In 1633, James published an account of his expedition: The Strange and Dangerous Voyage of Captain Thomas James. ... Thomas James was the fourth explorer to winter in that vast, inland sea; he was preceded by Henry Hudson (1610-11), Thomas Button (1612-13), and Jens Munk (1619-20). If we compare James with his predecessors, he stands up very well. He explored more miles of coastline than any of the others. And being a thoughtful and experienced leader of men, he did not suffer the dissension that wracked Hudson's crew. Nor did he suffer the frightful mortality that almost wiped out the Button and Munk expeditions. Munk lost 61 of the total complement of 64 men who sailed with him. James, in contrast, lost only six out of a crew of 22 men - two to accident, and four to scurvy
X-ray emission from young stars in Taurus-Auriga-Perseus: Luminosity functions and the rotation-activity-age relation
We report on a systematic search for X-ray emission from pre-main sequence
and young main sequence stars in the Taurus-Auriga-Perseus region. Our stellar
sample consists of all T Tauri stars from the Taurus-Auriga region, and all
late-type stars from the Pleiades and Hyades clusters which have been observed
by the ROSAT PSPC in pointed observations. We present the X-ray parameters for
all observed stars in tables, and study the connection between coronal X-ray
activity and stellar parameters for different subgroups of our sample. In
particular we compile X-ray luminosity functions (XLF), and discuss the
relations between X-ray emission and spectral type, age, and rotation, on the
largest sample so far.Comment: 19 pages, plus 6 tables, accepted for publication in A&
Ultra low-mass star and substellar formation in sigma Orionis
The nearby young sigma Orionis cluster (~360 pc, ~3 Ma) is becoming one of
the most important regions for the study of ultra low-mass star formation and
its extension down to the mass regimes of the brown dwarfs and planetary-mass
objects. Here, I introduce the sigma Orionis cluster and present three studies
that the JOVIAN group is developing: a pilot programme of near-infrared
adaptive-optics imaging, intermediate-resolution optical spectroscopy of a
large sample of stars of the cluster and a study of the mass function down to
the planetary-mass domain. This paper is a summary of the content of four
posters that I presented in the Ultra low-mass star formation and evolution
Workshop, as single author or on behalf of different collaborations.Comment: 4 pages, 4 figures. Proceeding at the IAC-TNG Workshop on Ultra
low-mass star formation and evolution, 28 June - 1 July 2005, La Palma,
Canary Islands, Spain. Accepted for publication in Astron. Nach
On the mass segregation of stars and brown dwarfs in Taurus
We use the new minimum spanning tree (MST) method to look for mass
segregation in the Taurus association. The method computes the ratio of MST
lengths of any chosen subset of objects, including the most massive stars and
brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in
the cluster. This mass segregation ratio (Lambda_MSR) enables a quantitative
measure of the spatial distribution of high-mass and low-mass stars, and brown
dwarfs to be made in Taurus.
We find that the most massive stars in Taurus are inversely mass segregated,
with Lambda_MSR = 0.70 +/- 0.10 (Lambda_MSR = 1 corresponds to no mass
segregation), which differs from the strong mass segregation signatures found
in more dense and massive clusters such as Orion. The brown dwarfs in Taurus
are not mass segregated, although we find evidence that some low-mass stars
are, with an Lambda_MSR = 1.25 +/- 0.15. Finally, we compare our results to
previous measures of the spatial distribution of stars and brown dwarfs in
Taurus, and briefly discuss their implications.Comment: 10 pages, 8 figures, accepted for publication in MNRA
X-ray Emission near the Substellar Limit: The sigma Orionis and Taurus Star Forming Regions
We have carried out an extensive search for X-ray emission from young, very
low-mass objects near and beyond the substellar limit, making use of archived
ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf
candidates in the young sigma Orionis and Taurus-Auriga associations. In sigma
Ori we identify three Brown Dwarf candidates with X-ray sources; in
Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list
published earlier. We combine this data with all previously X-ray detected
Brown Dwarfs and Brown Dwarf candidates in young stellar associations and star
forming regions to perform a study of stellar activity parameters on the as yet
largest sample of young, very low mass objects. A similar relation between
X-ray and bolometric luminosity, and H-alpha emission, respectively, as is
known for T Tauri stars seems to hold for young objects down to the substellar
limit, too. No signs for a change in X-ray activity are found on the transition
to substellar masses.Comment: 9 pages, 5 (9) figures; fig. 1-4 are available only in the preprint
or in the ps-file via
, accepted for
publication in A&
Orbital motion in T Tauri binary systems
Using speckle-interferometry we have carried out repeated measurements of
relative positions for the components of 34 T Tauri binary systems. The
projected separation of these components is low enough that orbital motion is
expected to be observable within a few years. In most cases orbital motion has
indeed been detected. The observational data is discussed in a manner similar
to Ghez et al. (1995). However, we extend their study to a larger number of
objects and a much longer timespan. The database presented in this paper is
valuable for future visible orbit determinations. It will yield empirical
masses for T Tauri stars that now are only poorly known. The available data is
however not sufficient to do this at the present time. Instead, we use short
series of orbital data and statistical distributions of orbital parameters to
derive an average system mass that is independent of theoretical assumptions
about the physics of PMS stars. For our sample this mass is 2.0 solar masses
and thus in the order of magnitude one expects for the mass sum of two T Tauri
stars. It is also comparable to mass estimates obtained for the same systems
using theoretical PMS evolutionary models.Comment: Accepted by Astronomy and Astrophysic
No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?
We report on the analysis of 2MASS near-infrared data of a sample of low-mass
stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster
membership have been spectroscopically confirmed using the Li I spectral line.
We find little evidence in the JHKs colour-colour diagram for near-infrared
excess emission for these cluster members. By comparison with model
expectations, at most 2 out of 34 stars show (H-K) colour consistent with a
near-infrared excess. This scarcity of near-infrared signatures of
circumstellar disks in the lower-mass and substellar regimes of this cluster
contrasts with findings in younger clusters, hinting at an age dependence of
the disk frequency. Taking into account the apparent cluster age, our result
supports the idea of a relatively fast (few Myr) disk dissipation and extends
this conclusion to the substellar regime. We also find some evidence that, in
this cluster, the disk frequency as measured by the Ks-band excess may be mass
dependent.Comment: 4 pages, 2 eps figures, accepted by Astronomy and Astrophysics
Letter
Star formation history and environment of the dwarf galaxy UGCA 92
We present a quantitative star formation history of the nearby dwarf galaxy
UGCA 92. This irregular dwarf is situated in the vicinity of the Local Group of
galaxies in a zone of strong Galactic extinction (IC 342 group of galaxies).
The galaxy was resolved into stars with HST/ACS including old red giant branch.
We have constructed a model of the resolved stellar populations and measured
the star formation rate and metallicity as function of time. The main star
formation activity period occurred about 8 - 14 Gyr ago. These stars are mostly
metal-poor, with a mean metallicity [Fe/H] ~ -1.5 -- -2.0 dex. About 84 per
cent of the total stellar mass was formed during this event. There are also
indications of recent star formation starting about 1.5 Gyr ago and continuing
to the present. The star formation in this event shows moderate enhancement
from ~ 200 Myr to 300 Myr ago. It is very likely that the ongoing star
formation period has higher metallicity of about -0.6 -- -0.3 dex. UGCA 92 is
often considered to be the companion to the starburst galaxy NGC 1569.
Comparing our star formation history of UGCA 92 with that of NGC 1569 reveals
no causal or temporal connection between recent star formation events in these
two galaxies. We suggest that the starburst phenomenon in NGC 1569 is not
related to the galaxy's closest dwarf neighbours and does not affect their star
formation history.Comment: 11 pages, 9 figures, accepted to MNRA
FLAMES spectroscopy of low-mass stars in the young clusters sigma Ori and lambda Ori
Aims. We performed a detailed membership selection and studied the accretion
properties of low-mass stars in the two apparently very similar young (1-10
Myr) clusters sigma Ori and lambda Ori.
Methods. We observed 98 and 49 low-mass (0.2-1.0 M_sun) stars in sigma Ori
and lambda Ori respectively, using the multi-object optical spectrograph FLAMES
at the VLT, with the high-resolution (R=17,000) HR15N grating (6470-6790 AA).
We used radial velocities, Li and Halpha to establish cluster membership and
Halpha and other optical emission lines to analyze the accretion properties of
members.
Results. We identified 65 and 45 members of the sigma Ori and lambda Ori
clusters, respectively and discovered 16 new candidate binary systems. We also
measured rotational broadening for 20 stars and estimated the mass accretion
rates in 25 stars of the sigma Ori cluster, finding values between 10^-11 and
10^-7.7 M_sun yr^-1 and in 4 stars of the lambda Ori cluster, finding values
between 10^-11 and 10^-10.1 M_sun yr-1. Comparing our results with the infrared
photometry obtained by the Spitzer satellite, we find that the fraction of
stars with disks and the fraction of active disks is larger in the sigma Ori
cluster (52+-9% and 78+-16%) than in lambda Ori (28+-8% and 40+-20%)
Conclusions. The different disk and accretion properties of the two clusters
could be due either to the effect of the high-mass stars and the supernova
explosion in the lambda Ori cluster or to different ages of the cluster
populations. Further observations are required to draw a definitive conclusion.Comment: 14 pages, 9 figures, 9 tables, accepted for publications in A&
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