52 research outputs found
A deep photometric survey of the eta Chamaeleontis cluster down to the brown dwarf - planet boundary
We report the outcome of the deep optical/infrared photometric survey of the
central region (33 X 33 arcmin or 0.9 pc^2) of the eta Chamaeleontis pre-main
sequence star cluster. The completeness limits of the photometry are I = 19.1,
J = 18.2 and H = 17.6; faint enough to reveal low mass members down to the
brown dwarf and planet boundary of ~ 13 M_Jup. We found no such low mass
members in this region. Our result combined with a previous shallower (I = 17)
but larger area survey indicates that low mass objects (0.013 < M/M(solar mass)
< 0.075) either were not created in the eta Cha cluster or were lost due to the
early dynamical history of the cluster and ejected to outside the surveyed
areas.Comment: 5 pages with 4 figures, accepted by MNRA
Population and dynamical state of the eta Chamaeleontis sparse young open cluster
We report new results in our continuing study of the unique compact (1 pc
extent), nearby (d = 97 pc), young (t = 9 Myr) stellar cluster dominated by the
B9 star eta Chamaeleontis. An optical photometric survey spanning 1.3 x 1.3 pc
adds two M5-M5.5 weak-lined T Tauri (WTT) stars to the cluster inventory which
is likely to be significantly complete for primaries with masses > 0.15 M_sun.
The cluster now consists of 17 primaries and approximately 9 secondaries lying
within 100 AU of their primaries. The apparent distribution of 9:7:1
single:binary:triple systems shows 2-4 x higher multiplicity than in the field
main sequence stars, and is comparable to that seen in other pre-main sequence
(PMS) populations. The initial mass function (IMF) is consistent with that of
rich young clusters and field stars. By extending the cluster IMF to lower
masses, we predict 10-14 additional low mass stars with 0.08 < M < 0.15 M-sun
and 10-15 brown dwarfs with 0.025 < M < 0.08 M_sun remain to be discovered. The
eta Cha cluster extends the established stellar density and richness
relationship for young open clusters. The radial distribution of stars is
consistent with an isothermal sphere, but mass segregation is present with > 50
percent of the stellar mass residing in the inner 6 arcmin (0.17 pc).
Considering that the eta Cha cluster is sparse, diffuse and young, the cluster
may be an ideal laboratory for distinguishing between mass segregation that is
primordial in nature, or arising from dynamical interaction processes.Comment: 10 pages, 7 figures, 2 tables, accepted for MNRA
Infrared study of the eta Chamaeleontis cluster and the longevity of circumstellar discs
We have analyzed JHKL observations of the stellar population of the ~9
Myr-old eta Chamaeleontis cluster. Using infrared (IR) colour-colour and
colour-excess diagrams, we find the fraction of stellar systems with near-IR
excess emission is 0.60 pm 0.13 (2_sigma). This results implies considerably
longer disc lifetimes than found in some recent studies of other young stellar
clusters. For the classical T Tauri (CTT) and weak-lined T Tauri (WTT) star
population, we also find a strong correlation between the IR excess and H_alpha
emission. The IR excesses of these stars indicate a wide range of star-disc
activity; from a CTT star showing high levels of accretion, to CTT - WTT
transition objects with evidence for some on-going accretion, and WTT stars
with weak or absent IR excesses. Of the 15 known cluster members, 4 stars with
IR excesses delta(K-L) > 0.4 mag are likely experiencing on-going accretion
owing to strong or variable optical emission. The resulting accretion fraction
(0.27 pm 0.13; 2_sigma) shows that the accretion phase, in addition to the
discs themselves, can endure for at least ~10 Myr.Comment: 7 pages, 4 figures, accepted for MNRA
Spectrophotometric properties of pre-main sequence stars: the Epsilon Chamaeleontis cluster
We present a study of flux-calibrated low-resolution optical spectroscopy of
ten stars belonging to eight systems in the ~ 5 Myr-old Epsilon Chamaeleontis
(Eps Cha) pre-main-sequence (PMS) star cluster. Using synthetic broadband
colours, narrow-band continuum, atomic and molecular lines derived from the
spectra, we compare the Eps Cha stars to a slightly older PMS cluster, the ~ 8
Myr-old Eta Cha cluster, and to main-sequence dwarfs. Using synthetic VRI
colours and other indices, we find that the relationship between broadband
colours and spectroscopic temperature indicators for Eps Cha cluster members is
indistinguishable from that of Gyr-old dwarfs. This result is identical to that
found earlier in Eta Cha. Gravity-sensitive line indices place the cluster
between the giant and dwarf sequences, and there is clear evidence that Eps Cha
stars have lower surface gravity than Eta Cha stars. This result is consistent
with Eps Cha being the slightly younger PMS association, a few Myr younger
according to the Hertzsprung-Russell (HR) diagram placement of these two
clusters and comparison with PMS evolutionary grids. Late M-type Eps Cha
cluster members show a B-band flux excess of ~ 0.2 mag compared to observations
of standard dwarfs, which might be related to enhanced magnetic activity. A
similar level of excess B-band emission appears to be a ubiquitous feature of
low mass members of young stellar populations with ages less than a few hundred
Myr, a very similar timescale to the PMS phase of elevated relative X-ray
luminosity.Comment: 8 figures and 3 tables It accepted for publication in Monthly Notices
of the Royal Astronomical Societ
The First Very Long Baseline Interferometry Image of 44 GHz Methanol Maser with the KVN and VERA Array (KaVA)
We have carried out the first very long baseline interferometry (VLBI)
imaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a
millimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA
(KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI
Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in
imaging compact maser features with a synthesized beam size of 2.7
milliarcseconds x 1.5 milliarcseconds (mas). These features are detected at a
limited number of baselines within the length of shorter than approximately 650
km corresponding to 100 Mlambda in the uv-coverage. The central velocity and
the velocity width of the 44 GHz methanol maser are consistent with those of
the quiescent gas rather than the outflow traced by the SiO thermal line. The
minimum component size among the maser features is ~ 5 mas x 2 mas, which
corresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc.
The brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0
x 10^{10} K, which are higher than estimated lower limit from a previous Very
Large Array observation with the highest spatial resolution of ~ 50 mas. The 44
GHz class I methanol maser in IRAS 18151-1208 is found to be associated with
the MM2 core, which is thought to be less evolved than another millimeter core
MM1 associated with the 6.7 GHz class II methanol maser.Comment: 19 pages, 3 figure
The JCMT BISTRO Survey: multiwavelength polarimetry of bright regions in NGC 2071 in the far-infrared/submillimetre range, with POL-2 and HAWC+
Polarized dust emission is a key tracer in the study of interstellar medium and of star formation. The observed polarization, however, is a product of magnetic field structure, dust grain properties, and grain alignment efficiency, as well as their variations in the line of sight, making it difficult to interpret polarization unambiguously. The comparison of polarimetry at multiple wavelengths is a possible way of mitigating this problem. We use data from HAWC+ /SOFIA and from SCUBA-2/POL-2 (from the BISTRO survey) to analyse the NGC 2071 molecular cloud at 154, 214, and 850 μm. The polarization angle changes significantly with wavelength over part of NGC 2071, suggesting a change in magnetic field morphology on the line of sight as each wavelength best traces different dust populations. Other possible explanations are the existence of more than one polarization mechanism in the cloud or scattering from very large grains. The observed change of polarization fraction with wavelength, and the 214-to-154 μm polarization ratio in particular, are difficult to reproduce with current dust models under the assumption of uniform alignment efficiency. We also show that the standard procedure of using monochromatic intensity as a proxy for column density may produce spurious results at HAWC+wavelengths. Using both long-wavelength (POL-2, 850 μm) and short-wavelength (HAWC+, ≲200μm) polarimetry is key in obtaining these results. This study clearly shows the importance of multi-wavelength polarimetry at submillimetre bands to understand the dust properties of molecular clouds and the relationship between magnetic field and star formation
- …