368 research outputs found
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
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
2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds: The north-eastern region of the Large Magellanic Cloud
We present spectral classifications from optical spectroscopy of 263 massive
stars in the north-eastern region of the Large Magellanic Cloud. The observed
two-degree field includes the massive 30 Doradus star-forming region, the
environs of SN1987A, and a number of star-forming complexes to the south of 30
Dor. These are the first classifications for the majority (203) of the stars
and include eleven double-lined spectroscopic binaries. The sample also
includes the first examples of early OC-type spectra (AAOmega 30 Dor 248 and
280), distinguished by the weakness of their nitrogen spectra and by C IV 4658
emission. We propose that these stars have relatively unprocessed CNO
abundances compared to morphologically normal O-type stars, indicative of an
earlier evolutionary phase. From analysis of observations obtained on two
consecutive nights, we present radial-velocity estimates for 233 stars, finding
one apparent single-lined binary and nine (>3sigma) outliers compared to the
systemic velocity; the latter objects could be runaway stars or large-amplitude
binary systems and further spectroscopy is required to investigate their
nature.Comment: Accepted by A&
The Conditions for Star Formation at Low Metallicity: Results from the LMC
We present our recent work on the conditions under which star formation
occurs in a metal-poor environment, the Large Magellanic Cloud ([Fe/H] ~ -0.4).
Water masers are used as beacons of the current star formation in HII regions.
Comparing their location with the dust morphology imaged with the Spitzer Space
Telescope, and additional Halpha imaging and groundbased near-infrared
observations, we conclude that the LMC environment seems favourable to
sequential star formation triggered by massive star feedback (Oliveira et al.
2006). Good examples of this are 30 Doradus and N 113. There are also HII
regions, such as N 105A, where feedback may not be responsible for the current
star formation although the nature of one young stellar object (YSO) suggests
that feedback may soon start making an impact. The chemistry in one YSO hints
at a stronger influence from irradiation effects in a metal-poor environment
where shielding by dust is suppressed (van Loon 2005)Comment: in "Triggered Star Formation in a Turbulent ISM", IAU symposium,
poster contribution; a better quality version of this manuscript can be found
at http://www.astro.keele.ac.uk/~jacco/papers/proc.ps a full version of the
poster can be found at
http://www.astro.keele.ac.uk/~jacco/papers/IAUS237_LMC_2006.pd
The M33 Variable Star Population Revealed by Spitzer
We analyze five epochs of Spitzer Space Telescope/Infrared Array Camera
(IRAC) observations of the nearby spiral galaxy M33. Each epoch covered nearly
a square degree at 3.6, 4.5, and 8.0 microns. The point source catalog from the
full dataset contains 37,650 stars. The stars have luminosities characteristic
of the asymptotic giant branch and can be separated into oxygen-rich and
carbon-rich populations by their [3.6] - [4.5] colors. The [3.6] - [8.0] colors
indicate that over 80% of the stars detected at 8.0 microns have dust shells.
Photometric comparison of epochs using conservative criteria yields a catalog
of 2,923 variable stars. These variables are most likely long-period variables
amidst an evolved stellar population. At least one-third of the identified
carbon stars are variable.Comment: Accepted for publication in ApJ. See published article for full
resolution figures and electronic table
The circumstellar envelope of AFGL 4106
We present new imaging and spectroscopy of the post-red supergiant binary
AFGL 4106. Coronographic imaging in H-alpha reveals the shape and extent of the
ionized region in the circumstellar envelope (CSE). Echelle spectroscopy with
the slit covering almost the entire extent of the CSE is used to derive the
physical conditions in the ionized region and the optical depth of the dust
contained within the CSE.
The dust shell around AFGL 4106 is clumpy and mixed with ionized gas. H-alpha
and [N II] emission is brightest from a thin bow-shaped layer just outside of
the detached dust shell. On-going mass loss is traced by [Ca II] emission and
blue-shifted absorption in lines of low-ionization species. A simple model is
used to interpret the spatial distribution of the circumstellar extinction and
the dust emission in a consistent way.Comment: 10 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysics Main Journa
An L'-band survey for circumstellar disks around low-mass stars in the young sigma Orionis cluster
We present new K- and L'-band imaging of a representative sample of members
of the young 3-5 Myr old sigma Orionis cluster. We identified objects with
(K-L') excess by analysing colour-colour diagrams and comparing the
observations with empirical main-sequence colours. The derived disk frequency
depends on the method used: (54 +/- 15)% if measured directly from the JHKL'
colour-colour diagram; or (46 +/-14)% if excesses are computed with respect to
predicted photospheric colours (according to the objects spectral types,
2-sigma excess detections). We compare the (K-L') excess with other indicators
and show that this is a robust and reliable disk indicator. We also compare the
derived disk frequency with similarly aged clusters and discuss possible
implications for disk lifetimes. The computed age of the sigma Ori cluster is
very important: a cluster age of 3 Myr would support the overall disk lifetime
of 6 Myr proposed in the literature, while an age > 4 Myr would point to a
slower disk destruction rate.Comment: MNRAS accepted, 11 pages, 6 figure
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