743 research outputs found
On the dispersion in lithium and potassium among late-type stars in young clusters: IC 2602
We have measured the equivalent width (EW) of the K I 7699 A line in a sample
of G and K-type members of the ~35 Myr old cluster IC 2602 for which a
dispersion in Li EWs had been reported by previous studies. Active cluster
stars with 0.75 < (B-V)o < 1 are characterized by a dispersion in the EW of the
K I 7699 A, while earlier and later-type stars do not show any significant
scatter. Cluster stars at all colors show potassium EW excesses with respect to
field inactive stars; furthermore, a statistically significant relationship is
found between differential potassium EWs and log Lx/Lbol ratios, indicating
that the EWs of the potassium feature are altered by activity. Our results
suggest that the dispersion in Li EWs observed among cluster stars later than
(B-V)o ~ 1 cannot be fully explained by the effects of activity. No final
conclusion can instead be drawn for earlier-type stars.Comment: accepted by A&
Lithium evolution in intermediate age and old open clusters: NGC 752 revisited
We present new high resolution spectroscopic observations of the intermediate
age (~2 Gyr) open cluster NGC 752. We investigate the Li vs. Teff distribution
and we obtain a new accurate determination of the cluster metallicity. We
compare the results for NGC 752 with other intermediate age and old clusters
spanning the age range from the Hyades (~0.6 Gyr) to NGC 188 (~6-8 Gyr). We
find that NGC 752 has a solar iron content ([Fe/H]=+0.01+/-0.04), at variance
with early reports of sub-solar metallicity. We find that NGC 752 is only
slightly more Li depleted than the younger Hyades and has a Li pattern almost
identical to that observed in the ~2 Gyr old IC 4651 and NGC 3680. As for the
latter clusters, we find that NGC 752 is characterized by a tight Li vs. Teff
distribution for solar-type stars, with no evidence for a Li spread as large as
the one observed in the solar age solar metallicity M 67. We discuss these
results in the framework of mixing mechanisms and Li depletion on the main
sequence (MS). We conclude that the development of a large scatter in Li
abundances in old open clusters might be an exception rather than the rule
(additional observations of old clusters are required), and that metallicity
variations of the order of ~0.2 dex do not affect Li depletion after the age of
the Hyades.Comment: A&A accepted, 10 pages, 5 ps figure
Rotation periods of late-type stars in the young open cluster IC 2602
We present the results of a monitoring campaign aimed at deriving rotation
periods for a representative sample of stars in the young (30 Myr) open cluster
IC 2602. Rotation periods were derived for 29 of 33 stars monitored. The
periods derived range from 0.2d (one of the shortest known rotation periods of
any single open cluster star) to about 10d (which is almost twice as long as
the longest period previously known for a cluster of this age). We are able to
confirm 8 previously known periods and derive 21 new ones, delineating the long
period end of the distribution. Despite our sensitivity to longer periods, we
do not detect any variables with periods longer than about 10d. The combination
of these data with those for IC 2391, an almost identical cluster, leads to the
following conclusions:
1) The fast rotators in a 30 Myr cluster are distributed across the entire
0.5 < B-V < 1.6 color range.
2) 6 stars in our sample are slow rotators, with periods longer than 6d.
3) The amplitude of variability depends on both the color and the period. The
dependence on the latter might be important in understanding the selection
effects in the currently available rotation period database and in planning
future observations.
4) The interpretation of these data in terms of theoretical models of
rotating stars suggests both that disk-interaction is the norm rather than the
exception in young stars and that disk-locking times range from zero to a few
Myr.Comment: 23 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Beryllium in turnoff stars of NGC6397: early Galaxy spallation, cosmochronology and cluster formation
We present the first detection of beryllium in two turnoff stars of the old,
metal-poor globular cluster NGC 6397. The beryllium lines are clearly detected
and we determine a mean beryllium abundance of log(Be/H)=-12.35 +/- 0.2. The
beryllium abundance is very similar to that of field stars of similar Fe
content. We interpret the beryllium abundance observed as the result of primary
spallation of cosmic rays acting on a Galactic scale, showing that beryllium
can be used as a powerful cosmochronometer for the first stellar generations.
With this method, we estimate that the cluster formed 0.2-0.3 Gyr after the
onset of star formation in the Galaxy, in excellent agreement with the age
derived from main sequence fitting. From the same spectra we also find low O
(noticeably different for the two stars) and high N abundances, suggesting that
the original gas was enriched in CNO processed material. Our beryllium results,
together with the N, O, and Li abundances, provide insights on the formation of
this globular cluster, showing that any CNO processing of the gas must have
occurred in the protocluster cloud before the formation of the stars we observe
now. We encounter, however, difficulties in giving a fully consistent picture
of the cluster formation, able to explain the complex overall abundance
pattern.Comment: to appear in A&
Evolution of lithium beyond the solar age: a Li survey of the old open cluster NGC 188
We have determined Li abundances for 11 G-type stars in the 6-8 Gyr old open
cluster NGC 188. These data significantly enlarge the number of cluster stars
with Li measurements, allowing us to extend the investigation of Li depletion
in open clusters to ages well beyond the age of the Sun. We have also inferred
the cluster metallicity which turns out to be solar. We find that solar-type
stars in NGC 188 are only slightly more Li depleted than the much younger
Hyades and no more Li depleted than stars of similar temperature in the 2-4 Gyr
younger cluster M 67. At variance with M 67, NGC 188 members show virtually no
scatter in their Li abundances. Surprisingly, no solar- type star in NGC 188
appears as Li depleted as the Sun or as the most Li depleted stars in M 67. We
discuss the implications of these results for mechanisms of internal mixing and
Li depletion in main sequence stars.Comment: to appear in A&
Classical T Tauri-like Outflow Activity in the Brown Dwarf Mass Regime
Over the last number of years spectroscopic studies have strongly supported
the assertion that protostellar accretion and outflow activity persists to the
lowest masses. In this paper we present the results of our latest investigation
of brown dwarf (BD) outflow activity and report on the discovery of two new
outflows. Here ISO-Oph 32 is shown to drive a blue-shifted outflow with a
radial velocity of 10-20 km/s and spectro-astrometric analysis constrains the
position angle of this outflow to 240 +/- 7 degrees. The BD candidate ISO-Cha1
217 is found to have a bipolar outflow bright in several key forbidden lines
(radial velocity = -20 km/s, +40 km/s) and with a PA of 190-210 degrees. A
striking feature of the ISO-Cha1 217 outflow is the strong asymmetry between
the red and blue-shifted lobes. This asymmetry is revealed in the relative
brightness of the two lobes (red-shifted lobe is brighter), the factor of two
difference in radial velocity (the red-shifted lobe is faster) and the
difference in the electron density (again higher in the red lobe). Such
asymmetries are common in jets from low mass protostars and the observation of
a marked asymmetry at such a low mass supports the idea that BD outflow
activity is scaled down from low mass protostellar activity.
In addition to presenting these new results, a comprehensive comparison is
made between BD outflow activity and jets launched by CTTSs. In particular, the
application of current methods for investigating the excitation conditions and
mass loss rates in CTT jets to BD spectra is explored.Comment: Accepted by Astrophysical Journa
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