137 research outputs found
Catalogue of candidate emission-line objects in the Small Magellanic Cloud
H\alpha and [O III] narrow band, wide field (7 * 7 degree), CCD images of the
Small Magellanic Cloud were compared and a catalogue of candidate planetary
nebulae and H\alpha emission-line stars was compiled. The catalogue contains
131 planetary nebulae candidates, 23 of which are already known to be or are
probable planetary nebulae or very low excitation objects. Also, 218
emission-line candidates have been identified with 113 already known. Our
catalogue therefore provides a useful supplement to those of Meyssonnier &
Azzopardi (1993) and Sanduleak, MacConnell & Davis Phillip (1978). Further
observations are required to confirm the identity of the unknown objects.Comment: 8 pages, accepted by MNRA
Spectrophotometric Libraries, Revised Photonic Passbands and Zero-points for UBVRI, Hipparcos and Tycho Photometry
We have calculated improved photonic passbands for the UBV RI, Hipparcos and
Tycho Hp,BT,VT standard systems using the extensive spectrophotometric
libraries of NGSL and MILES. Using the Hp passband, we adjusted the absolute
flux levels of stars in the spectrophotometric libraries so their synthetic Hp
magnitudes matched the precise Hipparcos catalog value. Synthetic photometry
based on the renormalized fluxes were compared to the standard UBVRI and BT, VT
magnitudes and revised synthetic zero-points were determined. The Hipparcos and
Tycho photometry system zero-points were also compared to the V magnitude
zero-points of the SAAO UBVRI system, the homogenized UBV system and the
Walraven V B system. The confusion in the literature concerning broadband
magnitudes, fluxes, passbands and the choice of appropriate mean wavelengths is
detailed and discussed in an appendix.Comment: 44 pages, including 16 figures and a 12 page appendi
First detection of a low-mass stellar halo around the young open cluster Eta Chamaeleontis
We have identified several lithium-rich low-mass (0.08<M<0.3 Msun) stars
within 5.5 deg of the young open cluster Eta Chamaeleontis, nearly four times
the radius of previous search efforts. Of these stars we propose 4 new probable
cluster members, and 3 possible members requiring further investigation. These
findings are consistent with a dynamical origin for the current configuration
of the cluster, without the need to invoke an abnormal Initial Mass Function
deficient in low-mass objects. Candidates were selected on the basis of DENIS
and 2MASS photometry, NOMAD astrometry and extensive follow-up spectroscopy.Comment: 5 Pages. 5 Figures and 1 Table. Accepted for publication in MNRAS
Letters. Higher resolution figures available at
http://www.mso.anu.edu.au/~murphysj/
Measuring the Balmer Jump and Effective Gravity in FGK Stars
It is difficult to accurately measure the effective gravity (log g) in
late-type stars using broadband (eg. UBV or SDSS) or intermediate-band (uvby)
photometric systems, especially when the stars can cover a range of
metallicities and reddenings. However, simple spectroscopic observational and
data reduction techniques can yield accurate values for log g through
comparison of the Balmer jumps of low-resolution spectra with recent grids of
synthetic flux spectra.Comment: 20 pages, 13 figures and 4 table
A constraint on the formation timescale of the young open cluster NGC 2264: Lithium abundance of pre-main sequence stars
The timescale of cluster formation is an essential parameter in order to
understand the formation process of star clusters. Pre-main sequence (PMS)
stars in nearby young open clusters reveal a large spread in brightness. If the
spread were considered as a result of a real spread in age, the corresponding
cluster formation timescale would be about 5 -- 20 Myr. Hence it could be
interpreted that star formation in an open cluster is prolonged for up to a few
tens of Myr. However, difficulties in reddening correction, observational
errors, and systematic uncertainties introduced by imperfect evolutionary
models for PMS stars, can result in an artificial age spread. Alternatively, we
can utilize Li abundance as a relative age indicator of PMS star to determine
the cluster formation timescale. The optical spectra of 134 PMS stars in NGC
2264 have been obtained with MMT/Hectochelle. The equivalent widths have been
measured for 86 PMS stars with a detectable Li line (3500 < T_eff [K] <= 6500).
Li abundance under the condition of local thermodynamic equilibrium (LTE) was
derived using the conventional curve of growth method. After correction for
non-LTE effects, we find that the initial Li abundance of NGC 2264 is A(Li) =
3.2 +/- 0.2. From the distribution of the Li abundances, the underlying age
spread of the visible PMS stars is estimated to be about 3 -- 4 Myr and this,
together with the presence of embedded populations in NGC 2264, suggests that
the cluster formed on a timescale shorter than 5 Myr.Comment: 53 pages, 12 figures, 4 tables, Accepted for publication in the
Astrophysical Journa
Episodic disk accretion in the halo of the 'old' Pre-Main Sequence cluster Eta Chamaeleontis
We present multi-epoch medium-resolution observations of two M4.5 candidate
members in the halo of the ~8 Myr Eta Chamaeleontis open cluster. Over six
months of observations both stars exhibited variations in their H-alpha line
profiles on timescales of days to months, with at least one episode of
substantial activity attributable to accretion from a circumstellar disk. We
derive an accretion rate ~10^-8.7 Msun/yr for this event, with a rate of
~10^-10.6 Msun/yr in quiescence. Episodic accretion like that observed here
means existing surveys of accreting Weak-lined T-Tauri Stars in young clusters
are likely incomplete and that gas dissipation timescales calculated from the
fraction of accreting objects are underestimates.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letter
- …