3,380 research outputs found
On the MBM12 Young Association
I present a comprehensive study of the MBM12 young association (MBM12A). By
combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS)
survey with new optical imaging and spectroscopy, I have performed a census of
the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X
1.4deg field encompassing the MBM12 cloud. I find five new members with masses
of 0.1-0.4 Msun and a few additional candidates that have not been observed
spectroscopically. From an analysis of optical and IR photometry for stars in
the direction of MBM12, I identify M dwarfs in the foreground and background of
the cloud. By comparing the magnitudes of these stars to those of local field
dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud;
it is not the nearest molecular cloud and is not inside the local bubble of hot
ionized gas as had been implied by previous distance estimates of 50-100 pc. I
have also used Li strengths and H-R diagrams to constrain the absolute and
relative ages of MBM12A and other young populations; these data indicate ages
of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations.
MBM12A may be a slightly evolved version of the aggregates of young stars
within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2
Myr).Comment: to be published in The Astrophysical Journal, 41 pages, 14 figures,
also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm
The Geological Context of Middle Pleistocene Crater Lake Deposits and Fossil Birds at Ulupau Head, Oahu, Hawaiian Islands
Els dipòsits pleistocènics Llacunars del cràter Ulupau, a Oahu, contenen les restes fòssils més antigues d' ocells conegudes a les Illes Hawaïi. Aquests fòssils són importants per documentar les taxes evolutives a una varietat de línies d'ocells terrestres i aquàtics de les illes. Lanàlísí estratigràfic detallat revela diferents cicles deposicionals que comporten dipòsits llacunars interestratificats, colluvium i paleosòls. La conca volcànica va ser oberta per l'erosió marina prèvia al pic del darrer interglacial (estadi isotòpic marí, o 'MIS' Se), fa 125 ka, restringint l'edat dels sediments del Llac al Pleistocè Mitjà. Els tres cicles de deposició (Unitats III, IV i V) semblen estar estretament lligats a pujades interglacials amb una ciclicitat de 100 ka. Donada la complexitat de la successió dels sediments del cràter i el temps requerit per a la seva formació, és probable que la conca deillac s'omplís al Llarg de diferents interglacials, assolint una extensió màxima de més de 50 ha durant l'estadi MIS 11, fa W1S 400 ka. Aquesta és també la màxima edat probable de les capes fossilíferes més riques de la Unitat III, però no es pot excloure que siguin del MIS 9 (fa 300 ka). Els requeriments d'hàbitat probables de diferents elements de l'avifauna donen llum sobre la paleoecologia de la localitat durant el Pleistocè Mitjà.Summary Pleistocene lake deposits in Ulupau Crater on Oahu contain the oldest fossil bird remains known from the Hawaiian Islands. These fossils are important for docwnenting evolutionary rates in a variety of lineages of land and water birds from the islands. Detailed stratigraphic analysis reveals distinct cycles of deposition involving interbedded lacustrine deposits, coLluviwn, and paleosols. The volcanic basin was breached by marine erosion prior to the peak of the last interglaciation (marine isotope stage or 'MIS' Se) over 125 ky ago, constraining the age of the lake sediments to the middle Pleistocene. The three cycles of deposition (Units III, Iv; and V) appear to be closely linked with interglacial highstands with 100 ky cyclicity. Given the complexity of the succession of crater sediments and the time required for their formation, it is probable that the lake basin filled over several interglacials, reaching a maximwn extent of over 50 ha during MIS 11, about 400 ky ago. This is also the likely maximum age of the richest fossil beds in Unit Iv; but MIS 9 (300 ky) also remains a possibility. Probable habitat requirements of the various elements of the avifauna provide insights into the paleoecology of the site during the middle Pleistocene
Discovery of seven T Tauri stars and a brown dwarf candidate in the nearby TW Hydrae Association
We report the discovery of five T Tauri star systems, two of which are
resolved binaries, in the vicinity of the nearest known region of recent star
formation, the TW Hydrae Association. The newly discovered systems display the
same signatures of youth (namely high X-ray flux, large Li abundance and strong
chromospheric activity) and the same proper motion as the original five
members. These similarities firmly establish the group as a bona fide T Tauri
association, unique in its proximity to Earth and its complete isolation from
any known molecular clouds.
At an age of ~10 Myr and a distance of ~50 pc, the association members are
excellent candidates for future studies of circumstellar disk dissipation and
the formation of brown dwarfs and planets. Indeed, as an example, our speckle
imaging revealed a faint, very likely companion 2" north of CoD-33 7795 (TWA
5). Its color and brightness suggest a spectral type ~M8.5 which, at an age of
~10^7 years, implies a mass ~20 M(Jupiter).Comment: 6 pages, 4 figures and 1 table. AAS LaTeX aas2pp4.sty. To be
published in Ap
Extinction Maps Toward The Milky Way Bulge: Two-Dimensional And Three-Dimensional Tests With APOGEE
Galactic interstellar extinction maps are powerful and necessary tools for Milky Way structure and stellar population analyses, particularly toward the heavily reddened bulge and in the midplane. However, due to the difficulty of obtaining reliable extinction measures and distances for a large number of stars that are independent of these maps, tests of their accuracy and systematics have been limited. Our goal is to assess a variety of photometric stellar extinction estimates, including both two-dimensional and three-dimensional extinction maps, using independent extinction measures based on a large spectroscopic sample of stars toward the Milky Way bulge. We employ stellar atmospheric parameters derived from high-resolution H-band Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra, combined with theoretical stellar isochrones, to calculate line-of-sight extinction and distances for a sample of more than 2400 giants toward the Milky Way bulge. We compare these extinction values to those predicted by individual near-IR and near+mid-IR stellar colors, two-dimensional bulge extinction maps, and three-dimensional extinction maps. The long baseline, near+mid-IR stellar colors are, on average, the most accurate predictors of the APOGEE extinction estimates, and the two-dimensional and three-dimensional extinction maps derived from different stellar populations along different sightlines show varying degrees of reliability. We present the results of all of the comparisons and discuss reasons for the observed discrepancies. We also demonstrate how the particular stellar atmospheric models adopted can have a strong impact on this type of analysis, and discuss related caveats.NSF Astronomy & Astrophysics Postdoctoral Fellowship AST-1203017Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA) PHY 08-22648U.S. National Science FoundationAlfred P. Sloan FoundationParticipating InstitutionsU.S. Department of Energy Office of Science ANR-12-BS05-0015-01Astronom
Overview of the spectrometer optical fiber feed for the Habitable-zone Planet Finder
The Habitable-zone Planet Finder (HPF) is a highly stabilized fiber fed
precision radial velocity (RV) spectrograph working in the Near Infrared (NIR):
810 - 1280 nm . In this paper we present an overview of the preparation of the
optical fibers for HPF. The entire fiber train from the telescope focus down to
the cryostat is detailed. We also discuss the fiber polishing, splicing and its
integration into the instrument using a fused silica puck. HPF was designed to
be able to operate in two modes, High Resolution (HR- the only mode mode
currently commissioned) and High Efficiency (HE). We discuss these fiber heads
and the procedure we adopted to attach the slit on to the HR fibers.Comment: Presented at 2018 SPIE Astronomical Telescopes + Instrumentation,
Austin, Texas, USA. 18 pages, 25 figures, and 2 table
A Prediction of Brown Dwarfs in Ultracold Molecular Gas
A recent model for the stellar initial mass function (IMF), in which the
stellar masses are randomly sampled down to the thermal Jeans mass from
hierarchically structured pre-stellar clouds, predicts that regions of
ultra-cold CO gas, such as those recently found in nearby galaxies by Allen and
collaborators, should make an abundance of Brown Dwarfs with relatively few
normal stars. This result comes from the low value of the thermal Jeans mass,
considering that the hierarchical cloud model always gives the Salpeter IMF
slope above this lower mass limit. The ultracold CO clouds in the inner disk of
M31 have T~3K and pressures that are probably 10 times higher than in the solar
neighborhood. This gives a mass at the peak of the IMF equal to 0.01 Msun, well
below the Brown Dwarf limit of 0.08 Msun. Using a functional approximation to
the IMF, the ultracold clouds would have 50% of the star-like mass and 90% of
the objects below the Brown Dwarf limit. The brightest of the Brown Dwarfs in
M31 should have an apparent, extinction-corrected K-band magnitude of ~21 mag
in their pre-main sequence phase.Comment: 13 pages, 2 figures, to be published in Astrophysical Journal, Vol
522, September 10, 199
Low Mass Stars and Substellar Objects in the NGC 1333 Molecular Cloud
We present the results of near-infrared imaging and low-resolution near-
infrared spectroscopy of low mass objects in the NGC 1333 molecular cloud. A
JHK survey of an 11.4' x 11.7' area of the northern cluster was conducted to a
sensitivity of K < 16 mag. Using near-infrared magnitudes and colors from this
and previously published surveys, twenty-five brown dwarf candidates were
selected toward the high extinction cloud core. Spectra in the K band were
obtained and comparisons of the depths of water vapor absorption bands in our
candidate objects with a grid of dwarf,subgiant, and giant standards were made
to derive spectral types. These data were then used to derive effective
temperatures and stellar luminosities which, when combined with theoretical
tracks and isochrones for pre-main sequence objects, resulted in estimates for
their masses and ages. The models suggest a median age for the sample of < 1
Myr with substellar masses for at least 9 of the candidates including the x-ray
flare source ASR 24. Surface gravities have been estimated for the brown dwarf
candidates and, for a given spectral type,found to resemble more closely dwarfs
than giants. Using the near-infrared imaging data and age estimates from the
spectroscopic sample, an extinction-limited sample in the northern cluster was
defined. Consistent with recent studies of other young clusters, this sample
exhibits an accretion disk frequency of 0.75 +-0.20 and a mass spectrum slope
across the hydrogen-burning limit of alpha < 1.6 where dN/dM ~ M^-(alpha).Comment: 22 postscript pages, 12 postscript figures, and 3 postscript tables.
Accepted for publication in the Astronomical Journal (February, 2004
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