The influence of the first stars on the formation of second-generation objects at high redshift may be determined, in part, by their metal enrichment of surrounding gas. At a critical metallicity, Z crit , primordial gas cools more efficiently by fine-structure lines of [C ii] (157.74 m), [O i] (63.18 m, 145.5 m), [Si ii] (34.8 m), and [Fe ii] (25.99 m, 35.35 m) than by H i or H 2 emission.This cooling may alter the process of fragmentation into smaller units. We study the time-dependent cooling of primordial gas enriched by heavy elements from early massive stars, particularly O, Si, and Fe. We define Z crit as the point when the total cooling rate by metals plus H 2 equals the adiabatic compressional heating. We explore two metallicity scenarios: (1) a single metallicity for all heavy elements and (2) individual metallicities (Z C , Z O , Z Si , and Z Fe ) from theoretical supernova yields. For dense gas (n ! 10 3 cm À3 ) with metals in relative solar abundances, fragmentation occurs at Z crit % 10 À3:5 Z . However, for lower density gas (n ¼ 1 100 cm À3 ), particularly in halos enriched in Si, O, and Fe, we find Z crit % 0:1% 1% Z . The critical metallicity approaches a minimum value at high n set by efficient LTE cooling, with thermalized level populations of fine-structure states and H 2 rotational states (J ¼ 2 and J ¼ 3). Primordial clouds of 10 8 M at 200 K are detectable in redshifted fine-structure lines, with far-infrared fluxes between 10 À22 and 10 À21 W m À2 . For metallicities Z O % 10 À3 and molecular fractions f H 2 % 10 À3 , the fine-structure emission lines of [O i], [Si ii], and [Fe ii] could be 10 2 -10 3 times stronger than the H 2 rotational lines at 28.22 m (J ¼ 2 ! 0) and 17.03 m (J ¼ 3 ! 1).
The EXtreme PREcision Spectrograph (EXPRES) is an optical fiber fed echelle instrument being designed and built at the Yale Exoplanet Laboratory to be installed on the 4.3-meter Discovery Channel Telescope operated by Lowell Observatory. The primary science driver for EXPRES is to detect Earth-like worlds around Sun-like stars. With this in mind, we are designing the spectrograph to have an instrumental precision of 15 cm/s so that the on-sky measurement precision (that includes modeling for RV noise from the star) can reach to better than 30 cm/s. This goal places challenging requirements on every aspect of the instrument development, including optomechanical design, environmental control, image stabilization, wavelength calibration, and data analysis. In this paper we describe our error budget, and instrument optomechanical design.
We calculate the cooling times at constant density for halos with virial temperatures from 100 K to 10^5 K that originate from a 3-sigma fluctuation of a CDM power spectrum in three different cosmologies. Our intention is to determine the first objects that can cool to low temperatures, but not to follow their dynamical evolution. We identify two generations of halos: those with low virial temperatures, Tvir < 9000 K that remain largely neutral, and those with larger virial temperatures that become ionized. The lower-temperature, lower-mass halos are the first to cool to 75 percent of their virial temperature. The precise temperature and mass of the first objects are dependent upon the molecular hydrogen (H2) cooling function and the cosmological model. The higher-mass halos collapse later but, in this paradigm, cool much more efficiently once they have done so, first via electronic transitions and then via molecular cooling: in fact, a greater residual ionization once the halos cool below 9000 K results in an enhanced H2 production and hence a higher cooling rate at low temperatures than for the lower-mass halos, so that within our constant-density model it is the former that are the first to cool to really low temperatures. We discuss the possible significance of this result in the context of CDM models in which the shallow slope of the initial fluctuation spectrum on small scales leads to a wide range of halo masses (of differing overdensities) collapsing over a small redshift interval. This ``crosstalk'' is sufficiently important that both high- and low-mass halos collapse during the lifetimes of the massive stars which may be formed at these epochs. Further investigation is thus required to determine which generation of halos plays the dominant role in early structure formation.Comment: 11 pages, 8 figures. Accepted for publication in MNRAS. Inclusion of Helium in the reaction networ
In February of 2014 the Yale Exoplanet Laboratory was commissioned to design, build, and deliver a high resolution (R = 60,000) spectrograph for the 1.65-meter telescope at the Molėtai Astronomical Observatory. The observatory is operated by the Institute of Theoretical Physics and Astronomy at Vilnius University. The Vilnius University Echelle Spectrograph (VUES) is a white-pupil design that is fed via an octagonal fiber from the telescope and has an operational bandpass from 400 to 880 nm. VUES incorporates a novel modular optomechanical design that allows for quick assembly and alignment on commercial optical tables. This approach allowed the spectrograph to be assembled and commissioned at Yale using lab optical tables and then reassembled at the observatory on a different optical table with excellent repeatability. The assembly and alignment process for the spectrograph was reduced to a few days, allowing the spectrograph to be completely disassembled for shipment to Lithuania, and then installed at the observatory during a 10-day period in June of 2015.
JPCam is a 14-CCD mosaic camera, using the new e2v 9k-by-9k 10m-pixel 16-channel detectors, to be deployed on a dedicated 2.55m wide-field telescope at the OAJ (Observatorio Astrofísico de Javalambre) in Aragon, Spain. The camera is designed to perform a Baryon Acoustic Oscillations (BAO) survey of the northern sky. The J-PAS survey strategy will use 54 relatively narrow-band (~13.8nm) filters equi-spaced between 370 and 920nm plus 3 broad-band filters to achieve unprecedented photometric red-shift accuracies for faint galaxies over ~8000 square degrees of sky. The cryostat, detector mosaic and read electronics is being supplied by e2v under contract to J-PAS while the mechanical structure, housing the shutter and filter assembly, is being designed and constructed by a Brazilian consortium led by INPE (Instituto Nacional de Pesquisas Espaciais). Four sets of 14 filters are placed in the ambient environment, just above the dewar window but directly in line with the detectors, leading to a mosaic having ~10mm gaps between each CCD. The massive 500mm aperture shutter is expected to be supplied by the Argelander-Institut für Astronomie, Bonn.We will present an overview of JPCam, from the filter configuration through to the CCD mosaic camera. A brief outline of the main J-PAS science projects will be included.
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