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

Oscillatory Magneto-Thermopower and Resonant Phonon Drag in a High-Mobility 2D Electron Gas

Experimental and theoretical evidence is presented for new low-magnetic-field ($B<5$ kG) 1/B-oscillations in the thermoelectric power of a high-mobility GaAs/AlGaAs two-dimensional (2D) electron gas. The oscillations result from inter-Landau-Level resonances of acoustic phonons carrying a momentum equal to twice the Fermi wavenumber at $B = 0$. Numerical calculations show that both 3D and 2D phonons can contribute to this effect.Comment: 4 pages, 5 figure

Impact of the new solar abundances on the calibration of the PMS binary system RS Cha

Context: In a recent work, we tried to obtain a calibration of the two components of the pre-main sequence binary system RS Cha by means of theoretical stellar models. We found that the only way to reproduce the observational parameters of RS Cha with standard stellar models is to decrease the initial abundances of carbon and nitrogen derived from the GN93 solar mixture of heavy elements by a few tenths of dex. Aims: In this work, we aim to reproduce the observational properties of the RS Cha stars with stellar evolution models based on the new AGS05 solar mixture recently derived from a three-dimensional solar model atmosphere. The AGS05 mixture is depleted in carbon, nitrogen and oxygen with respect to the GN93 mixture. Methods: We calculated new stellar models of the RS Cha components using the AGS05 mixture and appropriate opacity tables. We sought models that simultaneously satisfy the observations of the two components (masses, radii, luminosities, effective temperatures and metallicity). Results: We find that it is possible to reproduce the observational data of the RS Cha stars with AGS05 models based on standard input physics. From these models, the initial helium content of the system is Y~0.255 and its age is ~9.13 +- 0.12 Myr.Comment: Research note accepted in A&A, 5 pages, 2 figure

Stellar Populations of the Sagittarius Dwarf Irregular Galaxy

We present deep BVRI CCD photometry of the stars in the dwarf irregular galaxy SagDIG. The color-magnitude diagrams of the measured stars in SagDIG show a blue plume which consists mostly of young stellar populations, and a well-defined red giant branch (RGB). The foreground reddening of SagDIG is estimated to be E(B-V)=0.06. The tip of the RGB is found to be at I_(TRGB)=21.55 +/- 0.10 mag. From this the distance to this galaxy is estimated to be d = 1.18 +/- 0.10 Mpc. This result, combined with its velocity information, shows that it is a member of the Local Group. The mean metallicity of the red giant branch is estimated to be [Fe/H] < -2.2 dex. This shows that SagDIG is one of the most metal-poor galaxies. Total magnitudes of SagDIG (< r_H (= 107 arcsec)) are derived to be B^T=13.99 mag, V^T=13.58 mag, R^T=13.19 mag, and I^T=12.88 mag, and the corresponding absolute magnitudes are M_B=-11.62 mag, M_V=-11.97 mag, M_R=-12.33 mag, and M_I=-12.60 mag. Surface brightness profiles of the central part of SagDIG are approximately fit by a King model with a core concentration parameter c = log (r_t / r_c) ~ 0.6, and those of the outer part follow an exponential law with a scale length of 37 arcsec. The central surface brightness is measured to be mu_B (0) = 24.21 mag arcsec^(-2) and mu_V (0) =23.91 mag arcsec^(-2). The magnitudes and colors of the brightest blue and red stars in SagDIG (BSG and RSG) are measured to be, respectively, _BSG = 19.89 +/- 0.13 mag, _BSG = 0.08 +/- 0.07 mag, _RSG = 20.39 +/- 0.10 mag, and _RSG = 1.29 +/- 0.12 mag. The corresponding absolute magnitudes are derived to be _BSG = -5.66 mag and _RSG = -5.16 mag, which are about one magnitude fainter than those expected from conventional correlations with galaxy luminosity.Comment: 16 pages(AASLaTeX), 10 Postscript figures, Accepted for publication in Astronomical Journal, 200

Deep XMM-Newton observation of the Eta Chamaleontis cluster

The members of the Eta Chamaleontis cluster are in an evolutionary stage in which disks are rapidly evolving. It also presents some peculiarities, such as the large fraction of binaries and accretion disks, probably related with the cluster formation process. Its proximity makes this stellar group an ideal target for studying the relation between X-ray emission and those stellar parameters. The main objective of this work is to determine general X-ray properties of the cluster members in terms of coronal temperature, column density, emission measure, X-ray luminosity and variability. We also aim to establish the relation between the X-ray luminosity of these stars and other stellar parameters, such as binarity and presence of accretion disks. A study of flare energies for each flare event and their relation with some stellar parameters is also performed. We used proprietary data from a deep XMM-Newton observation pointed at the core of the Eta Chamaleontis cluster. Specific software for the reduction of XMM-Newton data was used for the analysis of our observation. For the detection of sources, we used the wavelet-based code PWDetect. General coronal properties were derived from plasma model fitting. We also determined variability of the Eta Chamaleontis members in the EPIC field-of-view. A total of six flare-like events were clearly detected in five different stars. For them, we derived coronal properties during the flare events and pseudo-quiescent state separately. In our observations, stars that underwent a flare event have higher X-ray luminosities in the pseudo-quiescent state than cluster members with similar spectral type with no indications of flaring, independently whether they have an accretion disk or not. Observed flare energies are typical of both pre-main and main-sequence M stars. We detected no difference between flare energies of stars with and without an accretion disk.Comment: Accepted for publication in Astronomy and Astrophysics. 12 pages, 30 individual figure

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

Stellar Populations and the Local Group Membership of the Dwarf Galaxy DDO 210

We present deep BVI CCD photometry of the stars in the dwarf galaxy DDO 210. The color-magnitude diagrams of DDO 210 show a well-defined red giant branch (RGB) and a blue plume. The tip of the RGB is found to be at I_TRGB = 20.95 +/- 0.10 mag. From this the distance to DDO 210 is estimated to be d = 950 +/- 50 kpc. The corresponding distance of DDO 210 to the center of the Local Group is 870 kpc, showing that it is a member of the Local Group. The mean metallicity of the red giant branch stars is estimated to be [Fe/H] = -1.9 +/- 0.1 dex. Integrated magnitudes of DDO 210 within the Holmberg radius (r_H=110 arcsec = 505 pc) are derived to be M_B=-10.6 +/- 0.1 mag and M_V=-10.9 +/- 0.1 mag. B and V surface brightness profiles of DDO 210 are approximately consistent with an exponential law with scale lengths r_s(B) = 161 pc and r_s(V) = 175 pc. The brightest blue and red stars in DDO 210 (BSG and RSG) are found to be among the faintest in the nearby galaxies with young stellar populations: _{BSG} = -3.41 +/- 0.11 mag and _{RSG} = -4.69 +/- 0.13 mag. An enhancement of the star formation rate in the recent past (several hundred Myrs) is observed in the central region of DDO 210. The opposite trend is observed in the outer region of the galaxy, suggesting a possible two-component structure of the kind disk/halo found in spiral galaxies. The real nature of this two-component structure must, however, be confirmed with more detailed observations.Comment: Latex file, 17 pages with 9 figures, uses emulateapj.sty To appear in the AJ (in August 1999

Correlations, compressibility, and capacitance in double-quantum-well systems in the quantum Hall regime

In the quantum Hall regime, electronic correlations in double-layer two-dimensional electron systems are strong because the kinetic energy is quenched by Landau quantization. In this article we point out that these correlations are reflected in the way the partitioning of charge between the two-layers responds to a bias potential. We report on illustrative calculations based on an unrestricted Hartree-Fock approximation which allows for spontaneous inter-layer phase coherence. The possibility of studying inter-layer correlations by capacitive coupling to separately contacted two-dimensional layers is discussed in detail.Comment: RevTex style, 21 pages, 6 postscript figures in a separate file; Phys. Rev. B (in press