Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo

We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red dwarfs, stars just massive enough to burn Hydrogen. The authors of this search (Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less than 6\% of the galactic halo. First, we extrapolate this result to include brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a $1/{\cal M}$ mass function. Then the total mass of red dwarfs and brown dwarfs is $\leq$18\% of the halo. This result is consistent with microlensing results assuming a popular halo model. However, using new stellar models and parallax observations of low mass, low metallicity stars, we obtain much tighter bounds on low mass stars. We find the halo red dwarf density to be $<1\%$ of the halo, while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the halo mass density of red dwarfs drops to 16-40 times less than the reported result of Bahcall et al (1994). For a $1/{\cal M}$ mass function, this suggests a total density of red dwarfs and brown dwarfs of $\sim$0.25-0.67\% of the halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would conflict with microlensing estimates by the \macho\ group (Alcock \etal 1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail To be published in ApJL. fig. 2 now available in postscript. Some minor changes in dealing with disk forground. Some cosmetic changes. Updated reference

The results of deep CCD field surveys: Very low mass halo population stars as dark matter

Halo and disk M dwarfs differ significantly in metallicity. Multi‐color deep CCD surveys are uniquely capable of detecting these separate populations of stars with differing metallicity and therefore colors. Analyzing very deep three‐band CCD images covering 192 arcmin2 at high galactic latitude we find no evidence for a population of extreme low mass M subdwarfs sufficient to account for the halo dark matter. These observations covering a volume of 2×105 pc3 are consistent with extrapolations of a halo luminosity function determined using low metallicity stars in the solar neighborhood. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87546/2/91_1.pd

The Mass-Function of Low Mass Halo Stars: Limits on Baryonic Halo Dark Matter

We derive mass functions (MF) for halo red dwarfs (the faintest hydrogen burning stars) and then extrapolate to place limits on the total mass of halo brown dwarfs (stars not quite massive enough to burn hydrogen). The mass functions are obtained from the luminosity function of a sample of 114 local halo stars in the USNO parallax survey (Dahn \etal 1995). We use stellar models of Alexander \etal (1996) and make varying assumptions about metallicity and about possible unresolved binaries in the sample. We find that the MF for halo red dwarfs cannot rise more quickly than $1/m^2$ as one approaches the hydrogen burning limit. Using recent results from star formation theory, we extrapolate the MF into the brown-dwarf regime. We see that likely extrapolations imply that the total mass of brown dwarfs in the halo is less than $\sim 3\%$ of the local mass density of the halo ($\sim 0.3\%$ for the more realistic models we consider). Our limits apply to brown dwarfs in the halo that come from the same stellar population as the red dwarfs.Comment: Significant changes over previous submission. To be published ApJ Letters, 16 pages, latex, one figur

The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability

We report on the methodology and first results from the Deep Lens Survey transient search. We utilize image subtraction on survey data to yield all sources of optical variability down to 24th magnitude. Images are analyzed immediately after acquisition, at the telescope and in near-real time, to allow for followup in the case of time-critical events. All classes of transients are posted to the web upon detection. Our observing strategy allows sensitivity to variability over several decades in timescale. The DLS is the first survey to classify and report all types of photometric and astrometric variability detected, including solar system objects, variable stars, supernovae, and short timescale phenomena. Three unusual optical transient events were detected, flaring on thousand-second timescales. All three events were seen in the B passband, suggesting blue color indices for the phenomena. One event (OT 20020115) is determined to be from a flaring Galactic dwarf star of spectral type dM4. From the remaining two events, we find an overall rate of \eta = 1.4 events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit of \eta < 4.3. One of these events (OT 20010326) originated from a compact precursor in the field of galaxy cluster Abell 1836, and its nature is uncertain. For the second (OT 20030305) we find strong evidence for an extended extragalactic host. A dearth of such events in the R passband yields an upper 95% confidence limit on short timescale astronomical variability between 19.5 < R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically variable objects, as well as an example of photometric variability with an undetected precursor.Comment: 24 pages, 12 figures, 3 tables. Accepted for publication in ApJ. Variability data available at http://dls.bell-labs.com/transients.htm

The Deep Lens Survey

The Deep Lens Survey (DLS) is a deep BVRz' imaging survey of seven 2x2 degree fields, with all data to be made public. The primary scientific driver is weak gravitational lensing, but the survey is also designed to enable a wide array of other astrophysical investigations. A unique feature of this survey is the search for transient phenomena. We subtract multiple exposures of a field, detect differences, classify, and release transients on the Web within about an hour of observation. Here we summarize the scientific goals of the DLS, field and filter selection, observing techniques and current status, data reduction, data products and release, and transient detections. Finally, we discuss some lessons which might apply to future large surveys such as LSST.Comment: to appear in Proc. SPIE Vol. 4836. v2 contains very minor change

The Chemical Composition of the 30 Doradus Nebula Derived from VLT Echelle Spectrophotometry

Echelle spectrophotometry of the 30 Doradus nebula in the LMC is presented. The data consists of VLT UVES observations in the 3100 to 10350 A range. The intensities of 366 emission lines have been measured, including 269 identified permitted lines of H 0, He 0, C 0, C+, N+, N++, O 0, O+, Ne 0, Ne+, S+, S++, Si 0 , Si+, Si++, Ar+, and Mg+. Electron temperatures and densities have been determined using different line intensity ratios. The He+, C++, O+, and O++ ionic abundances have been derived from recombination lines, these abundances are almost independent of the temperature structure of the nebula. Alternatively abundances from collisionally excited lines have been derived for a large number of ions of different elements, these abundances depend strongly on the temperature structure. Accurate t^2 values have been derived from the Balmer continuum, and by comparing the C++, O+, and O++ ionic abundances obtained from collisionally excited and recombination lines. The chemical composition of 30 Doradus is compared with those of Galactic and extragalactic HII regions. The values of Delta Y / Delta O, Delta Y / Delta Z, and Yp are also discussed.Comment: 37 pages, 2 figures, submitted to the Astrophysical Journa

Spatially resolved spectroscopy of planetary nebulae and their halos I. Five galactic disk objects

Strong mass loss off stars at the tip of the asymptotic giant branch (AGB) profoundly affects properties of these stars and their surroundings, including the subsequent planetary nebula (PN) stage. With this study we wanted to determine physical properties of mass loss by studying weakly emitting halos, focusing on objects in the galactic disk. Halos surround the, up to several thousand times, brighter central regions of PNe. Young halos, specifically, still contain information of the preceeding final mass loss stage on the AGB. In the observations we used the method of integral field spectroscopy with the PMAS instrument. This is the first committed study of halos of PNe that uses this technique. We improved our data analysis by a number of steps. In a study of the influence of scattered light we found that a moderate fraction of intensities in the inner halo originate in adjacent regions. As we combine line intensities of distant wavelengths, and because radial intensity gradients are steep, we corrected for effects of differential atmospheric refraction. In order to increase the signal-to-noise of weak emission lines we introduced a dedicated method to bin spectra of individual spatial elements. We also developed a general technique to subtract telluric lines - without using separate sky exposures. By these steps we avoided introducing errors of several thousand Kelvin to our temperature measurements in the halo. For IC3568 we detected a halo. For M2-2 we found a halo radius that is 2.5 times larger... (abridged)Comment: 27 pages, 29 figures, A&A (in press), Abridged abstract, Corrected and clarified various minor issues; the section on scattered light is significantly clarifie

Deep lens survey

The Deep Lens Survey (DLS) is a deep BV Rz' imaging survey of seven 2°×2° degree fields, with all data to be made public. The primary scientific driver is weak gravitational lensing, but the survey is also designed to enable a wide array of other astrophysical investigations. A unique feature of this survey is the search for transient phenomena. We subtract multiple exposures of a field, detect differences, classify, and release transients on the Web within about an hour of observation. Here we summarize the scientific goals of the DLS, field and filter selection, observing techniques and current status, data reduction, data products and release, and transient detections. Finally, we discuss some lessons which might apply to future large surveys such as LSST