2,801 research outputs found
The effect of starspots on the radii of low-mass pre-main sequence stars
A polytropic model is used to investigate the effects of dark photospheric
spots on the evolution and radii of magnetically active, low-mass (M<0.5Msun),
pre-main sequence (PMS) stars. Spots slow the contraction along Hayashi tracks
and inflate the radii of PMS stars by a factor of (1-beta)^{-N} compared to
unspotted stars of the same luminosity, where beta is the equivalent covering
fraction of dark starspots and N \simeq 0.45+/-0.05. This is a much stronger
inflation than predicted by the models of Spruit & Weiss (1986) for main
sequence stars with the same beta, where N \sim 0.2 to 0.3. These models have
been compared to radii determined for very magnetically active K- and M-dwarfs
in the young Pleiades and NGC 2516 clusters, and the radii of tidally-locked,
low-mass eclipsing binary components. The binary components and ZAMS K-dwarfs
have radii inflated by \sim 10 per cent compared to an empirical
radius-luminosity relation that is defined by magnetically inactive field
dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that
are still on their Hayashi tracks, are inflated by up to \sim 40 per cent. If
this were attributable to starspots alone, we estimate that an effective spot
coverage of 0.35 < beta < 0.51 is required. Alternatively, global inhibition of
convective flux transport by dynamo-generated fields may play a role. However,
we find greater consistency with the starspot models when comparing the loci of
active young stars and inactive field stars in colour-magnitude diagrams,
particularly for the highly inflated PMS stars, where the large, uniform
temperature reduction required in globally inhibited convection models would
cause the stars to be much redder than observed.Comment: MNRAS in press, 13 page
Rotation and activity in the solar-type stars of NGC 2547
We present high resolution spectroscopy of a sample of 24 solar-type stars in
the young (15-40 Myr), open cluster, NGC 2547. We use our spectra to confirm
cluster membership in 23 of these stars, determine projected equatorial
velocities and chromospheric activity, and to search for the presence of
accretion discs. We have found examples of both fast (vsini>50kms) and slow
(vsini<10kms) rotators, but find no evidence for active accretion in any of the
sample. The distribution of projected rotation velocities is indistinguishable
from the slightly older IC 2391 and IC 2602 clusters, implying similar initial
angular momentum distributions and circumstellar disc lifetimes. The presence
of very slow rotators indicates that either long (10-40 Myr) disc lifetimes or
internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602)
were born with more slowly rotating stars than are presently seen in even
younger clusters and associations. The solar-type stars in NGC 2547 follow a
similar rotation-activity relationship to that seen in older clusters. X-ray
activity increases until a saturation level is reached for vsini>15-20kms. We
are unable to explain why this saturation level, of log (L_x/L_bol)~-3.3, is a
factor of two lower than in other clusters, but rule out anomalously slow
rotation rates or uncertainties in X-ray flux calculations.Comment: Accepted by MNRA
Low Mass Stars and Brown Dwarfs around Sigma Orionis
We present optical spectroscopy of 71 photometric candidate low-mass members
of the cluster associated with Sigma Orionis. Thirty-five of these are found to
pass the lithium test and hence are confirmed as true cluster members, covering
a mass range of <0.055-0.3M_{sun}, assuming a mean cluster age of <5 Myr. We
find evidence for an age spread on the (I, I-J) colour magnitude diagram,
members appearing to lie in the range 1-7 Myr. There are, however, a
significant fraction of candidates that are non-members, including some
previously identified as members based on photometry alone. We see some
evidence that the ratio of spectroscopically confirmed members to photometric
candidates decreases with brightness and mass. This highlights the importance
of spectroscopy in determining the true initial mass-function.Comment: To appear in the 12th Cambridge Workshop on Cool Stars Stellar
Systems and the Su
Star Formation in the Eagle Nebula and NGC 6611
We present IZJHKL' photometry of the core of the cluster NGC 6611 in the
Eagle Nebula. This photometry is used to constrain the Initial Mass Function
(IMF) and the circumstellar disk frequency of the young stellar objects.
Optical spectroscopy of 258 objects is used to confirm membership and constrain
contamination as well as individual reddening estimates. Our overall aim is to
assess the influence of the ionizing radiation from the massive stars on the
formation and evolution of young low-mass stars and their disks. The disk
frequency determined from the JHKL' colour-colour diagram suggests that the
ionizing radiation from the massive stars has little effect on disk evolution
(Oliveira et al. 2005). The cluster IMF seems indistinguishable from those of
quieter environments; however towards lower masses the tell-tale signs of an
environmental influence are expected to become more noticeable, a question we
are currently addressing with our recently acquired ultra-deep (ACS and NICMOS)
HST images.Comment: in "Triggered Star Formation in a Turbulent ISM", IAU symposium,
poster contribution; a full version of the poster can be found at
http://www.astro.keele.ac.uk/~jacco/papers/IAUS237_Eagle_2006.pd
The low-mass Initial Mass Function in the young cluster NGC 6611
NGC 6611 is the massive young cluster (2-3 Myr) that ionises the Eagle
Nebula. We present very deep photometric observations of the central region of
NGC 6611 obtained with the Hubble Space Telescope and the following filters:
ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to
ground-based IZJH filters. This survey reaches down to I ~ 26 mag. We construct
the Initial Mass Function (IMF) from ~ 1.5 Msun well into the brown dwarf
regime (down to ~ 0.02 Msun). We have detected 30-35 brown dwarf candidates in
this sample. The low-mass IMF is combined with a higher-mass IMF constructed
from the groundbased catalogue from Oliveira et al. (2005). We compare the
final IMF with those of well studied star forming regions: we find that the IMF
of NGC 6611 more closely resembles that of the low-mass star forming region in
Taurus than that of the more massive Orion Nebula Cluster (ONC). We conclude
that there seems to be no severe environmental effect in the IMF due to the
proximity of the massive stars in NGC 6611.Comment: accepted for publication in MNRAS (main journal); 18 pages, 12
figures and 3 table
Cool stars in NGC 2547 and pre main sequence lithium depletion
We present the results of a spectroscopic survey of X-ray selected, low-mass
candidate members of the young open cluster NGC 2547. Using a combination of
photometry, spectroscopic indices and radial velocities we refine our candidate
list and then use our spectroscopy to study the progression of lithium
depletion in low-mass pre main sequence stars. We derive lithium abundances or
upper limits for all our candidate members, which have effective temperatures
in the range 5000>Teff>3200K, and compare these with predictions for lithium
burning and depletion provided by a number of models and also with the lithium
depletion seen in younger and older stars. We find that some models can
reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age
of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the
Hertzsprung-Russell diagram. But the lack of significant further lithium
depletion between NGC 2547 and older clusters argues for an age of at least
50Myr, more in keeping with the lack of lithium observed in even fainter NGC
2547 candidates. We show that reconciliation of these age estimates may require
additions to the physics incorporated in current generations of pre main
sequence models.Comment: Accepted for publication in MNRAS (better version of Fig.1 available
at http://www.astro.keele.ac.uk/~rdj/
Ages, Distances, and the Initial Mass Functions of Stellar Clusters
We provide a review of the current status of several topics on the ages,
distances, and mass functions of open clusters, with a particular emphasis on
illuminating the areas of uncertainty. Hipparcos has obtained parallaxes for
nearby open clusters that have expected accuracies much better than has been
previously achievable. By using the lithium depletion boundary method and
isochrone fitting based on much improved new theoretical evolutionary models
for low mass stars, it is arguable that we will soon have have much better age
scales for clusters and star-forming regions. With improved optical and near-IR
cameras, we are just now beginning to extend the mass function of open clusters
like the Pleiades into the regime below the hydrogen burning mass limit.
Meanwhile, observations in star-forming regions are in principle capable of
identifying objects down to of order 10 Jupiter masses.Comment: 13 pages, including 3 embedded figures (4 EPS files). To appear in
"11th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun," ed. R.
J. Garcia Lopez, R. Rebolo, and M. R. Zapatero Osori
An XMM-Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr
We report XMM-Newton observations of the young open cluster NGC 2547 which
allow us to characterise coronal activity in solar-type stars at an age of 30
Myr. X-ray emission peaks among G-stars at luminosities (0.3-3keV) of
Lx~10^{30.5} erg/s and declines to Lx<=10^{29.0} erg/s among M-stars. Coronal
spectra show evidence for multi-temperature differential emission measures and
low coronal metal abundances (Z~0.3). The G- and K-type stars follow the same
relationship between X-ray activity and Rossby number established in older
clusters and field stars, although most solar-type stars in NGC 2547 exhibit
saturated/super-saturated X-ray activity levels. Median levels of Lx and
Lx/Lbol in the solar-type stars of NGC 2547 are similar to T-Tauri stars of the
Orion Nebula cluster (ONC), but an order of magnitude higher than in the older
Pleiades. The spread in X-ray activity levels among solar-type stars in NGC
2547 is much smaller than in older or younger clusters. Coronal temperatures
increase with Lx, Lx/Lbol and surface X-ray flux. Active solar-type stars in
NGC 2547 have coronal temperatures between those in the ONC and the most active
older ZAMS stars. A flaring rate (for total flare energies [0.3-3keV] >10^{34}
erg) of 1 every 350^{+350}_{-120} ks was found for solar-type stars, similar to
rates found in the ONC and Pleiades. Comparison with ROSAT HRI data taken 7
years previously reveals that only 10-15 percent of solar-type stars or stars
with Lx>3x10^{29} erg/s exhibit X-ray variability by more than a factor of two.
The similar levels of X-ray activity and rate of occurrence for large flares in
NGC 2547 and the ONC demonstrate that the X-ray radiation environment around
young solar-type stars remains relatively constant over their first 30 Myr
(abridged).Comment: Accepted for publication in MNRAS. Electronic tables available from
the autho
The Li Overabundance of J37: Diffusion or Accretion?
In September 2002 the discovery of a super Li-rich F-dwarf (J37) in NGC 6633,
an iron poor analogue of the better studied Hyades and Praecepe open clusters,
was announced. This unique star was thought to be the smoking gun for the
action of diffusion, models of which predict a narrow "Li-peak" at
approximately the correct temperature. However, with more detailed studies into
J37s abundance pattern this star provides firm evidence for the accretion of
planetesimals or other material from the circumstellar environment of new born
stars.
Thanks to the specific predictions made about the behaviour of Be abundances,
(the most striking of which being no Be in super-Li-rich dwarfs subject to
diffusion) the opposing diffusion/accretion predictions can be tested.
Initial modelling of the Be line indicates that J37 is as Be rich as it is Li
rich; log N(Be) = 2.25 +/- 0.25, and so is broadly consistent with an
accretion-fuelled enhancement. However, that both Li and Be are enhanced by
much more than the iron-peak elements (as determined in previous studies)
suggests that diffusion also plays a role in increasing the abundances of Li
and Be specifically.
Furthermore, a new data set from the UVES/UT2 combination has allowed the
elemental abundance of Iron to be measured, and the set of preliminary stellar
parameters determined; Teff ~ 7340 K, log g ~ 4.1, microturbulence ~ 4.3 km/s,
[Fe/H] ~ 0.50. This again provides distinct evidence for the effects of
accretion in J37 and requires a new synthesis of the Be doublet.Comment: 5 pages, 2 figures. Poster presented at IAU Symposium 224 "The A Star
Puzzle", 7-13 July 2004, Poprad, Slovaki
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