Abundance and Sizes of Bay Scallops in Heterogeneous Habitats Along the Gulf Coast of Florida

Southern bay scallops (Argopecten irradians concentricus) form the basis of a recreational fishery along Florida\u27s Gulf Coast. Recent declines in scallop abundances have led to significant harvest restrictions. As a way to gain insight into influences on scallop abundances and size, surveys of bay scallops and coastal habitats were conducted in two relatively undisturbed, shallow estuaries along the north-central Gulf Coast of Florida. Scallop abundances did not vary significantly between years or between locations kilometers apart. Shell heights did vary significantly between years at locations kilometers apart; however, these differences were not consistently related to differences in chlorophyll concentrations in the water column or distributions of benthic habitat classes. At the 100-m scale within locations, scallops were not proportionally distributed across the major habitat classes (i.e., Syringodium filiforme, Thalassia testudinum, mixed seagrass assemblage, other seagrasses, and areas of no/low seagrass cover). In general, proportionately more scallops were observed in association with S. filiforme, T. testudinum, and mixed seagrass habitats. Bay scallops collected from S. filiforme and areas of no/low grass cover were consistently 1-3 mm larger than those collected from T. testudinum and mixed seagrass assemblages. These results suggest the importance of S. filiforme and T. testudinum as habitats for bay scallops. The results also point to the need for further investigation into possible functional differences among seagrass species that may influence the ecology of bay scallops at a small spatial scale and the need for closer examination of scallop movement that may allow for active habitat selection. The work presented here, plus further efforts to elucidate the drivers of small-scale differences in scallop abundances and sizes, will benefit managers who seek to enhance scallop fisheries or protect and restore coastal habitats

Discovery of 9 Ly alpha emitters at redshift z~3.1 using narrow-band imaging and VLT spectroscopy

Narrow-band imaging surveys aimed at detecting the faint emission from the 5007 [O III] line of intracluster planetary nebulae in Virgo also probe high redshift z=3.1 Ly alpha emitters. Here we report on the spectroscopic identification of 9 Ly alpha emitters at z=3.13, obtained with the FORS spectrograph at Unit 1 of the ESO Very Large Telescope (VLT UT1). The spectra of these high redshift objects show a narrow, isolated Ly alpha emission with very faint (frequently undetected) continuum, indicating a large equivalent width. No other features are visible in our spectra. Our Ly alpha emitters are quite similar to those found by Hu (1998), Cowie & Hu (1998) and Hu et al. (1998). Using simple population synthesis models, on the assumption that these sources are regions of star formation, we conclude that the nebulae are nearly optically thick and must have a very low dust content, in order to explain the high observed Ly alpha equivalent widths. For the cosmological and star formation parameters we adopted, the total stellar mass produced would seem to correspond to the formation of rather small galaxies, some of which are perhaps destined to merge. The implied star formation density in our sampled comoving volume is probably somewhat smaller than, but of the same order of magnitude as the star formation density at z=3 derived by other authors from Lyman-break galaxy surveys. This result agrees with the expectation that the Ly alpha emitters are a low-metallicity (or low-dust) tail in a distribution of star forming regions at high redshifts. Finally, the Ly alpha emitters may contribute as many H-ionizing photons as QSOs at z=3.Comment: 26 pages, 17 Postscript figures, ApJ in pres

On Pulsar Distance Measurements and their Uncertainties

Accurate distances to pulsars can be used for a variety of studies of the Galaxy and its electron content. However, most distance measures to pulsars have been derived from the absorption (or lack thereof) of pulsar emission by Galactic HI gas, which typically implies that only upper or lower limits on the pulsar distance are available. We present a critical analysis of all measured HI distance limits to pulsars and other neutron stars, and translate these limits into actual distance estimates through a likelihood analysis that simultaneously corrects for statistical biases. We also apply this analysis to parallax measurements of pulsars in order to obtain accurate distance estimates and find that the parallax and HI distance measurements are biased in different ways, because of differences in the sampled populations. Parallax measurements typically underestimate a pulsar's distance because of the limited distance to which this technique works and the consequential strong effect of the Galactic pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading to overestimated distances because the bright pulsars on which this technique is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the Astrophysical Journa

Simulated LSST Survey of RR Lyrae Stars throughout the Local Group

We report on a study to determine the efficiency of the Large Synoptic Survey Telescope (LSST) to recover the periods, brightnesses, and shapes of RR Lyrae stars' light curves in the volume extending to heliocentric distances of 1.5 Mpc. We place the smoothed light curves of 30 type ab and 10 type c RR Lyrae stars in 1007 fields across the sky, each of which represents a different realization of the LSST sampling cadences, and that sample five particular observing modes. A light curve simulation tool was used to sample the idealized RR Lyrae stars' light curves, returning each as it would have been observed by LSST, including realistic photometric scatter, limiting magnitudes, and telescope downtime. We report here the period, brightness, and light curve shape recovery as a function of apparent magnitude and for survey lengths varying from 1 to 10 years. We find that 10 years of LSST data are sufficient to recover the pulsation periods with a fractional precision of ~10^(–5) for ≥90% of ab stars within ≈360 kpc of the Sun in Universal Cadence fields and out to ≈760 kpc for Deep Drilling fields. The 50% completeness level extends to ≈600 kpc and ≈1.0 Mpc for the same fields, respectively. For virtually all stars that had their periods recovered, their light curve shape parameter φ_31 was recovered with sufficient precision to also recover photometric metallicities to within 0.14 dex (the systematic error in the photometric relations). With RR Lyrae stars' periods and metallicities well measured to these distances, LSST will be able to search for halo streams and dwarf satellite galaxies over half of the Local Group, informing galaxy formation models and providing essential data for mapping the Galactic potential. This study also informs the LSST science operations plan for optimizing observing strategies to achieve particular science goals. We additionally present a new [Fe/H]-φ_31 photometric relation in the r band and a new and generally useful metric for defining period recovery for time domain surveys

Global Health and Economic Impacts of Future Ozone Pollution

Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).We assess the human health and economic impacts of projected 2000-2050 changes in ozone pollution using the MIT Emissions Prediction and Policy Analysis-Health Effects (EPPA-HE) model, in combination with results from the GEOS-Chem global tropospheric chemistry model that simulated climate and chemistry effects of IPCC SRES emissions. We use EPPA to assess the human health damages (including acute mortality and morbidity outcomes) caused by ozone pollution and quantify their economic impacts in sixteen world regions. We compare the costs of ozone pollution under scenarios with 2000 and 2050 ozone precursor and greenhouse gas emissions (SRES A1B scenario). We estimate that health costs due to global ozone pollution above pre-industrial levels by 2050 will be $580 billion (year 2000$) and that acute mortalities will exceed 2 million. We find that previous methodologies underestimate costs of air pollution by more than a third because they do not take into account the long-term, compounding effects of health costs. The economic effects of emissions changes far exceed the influence of climate alone.United States Department of Energy, Office of Science (BER) grants DE-FG02-94ER61937 and DE-FG02-93ER61677, the United States Environmental Protection Agency grant EPA-XA-83344601-0, and the industrial and foundation sponsors of the MIT Joint Program on the Science and Policy of Global Change

Intracluster Red Giant Stars in the Virgo Cluster

We have used the WFPC2 camera of the Hubble Space Telescope to obtain deep F814W images of a blank field in the Virgo Cluster located 41 arcmin northwest of M87. We perform star counts in that field, and in another Virgo field observed by Ferguson, Tanvir & von Hippel (1998), and show that, when compared to the Hubble Deep Field North and South, the Virgo Cluster contains an excess of objects with magnitudes I > 27. We attribute this excess to a population of intracluster red-giant branch (IC-RGB) stars. By modeling the luminosity function of these stars, we show that the tip of the Virgo RGB is at I = 27.31 +0.27/-0.17 and that the cluster contains a small, but significant, excess of stars that are up to ~1 mag brighter than this tip. If this luminous component is due entirely to stars on the asymptotic giant branch (AGB), it implies an age for the population of > 2 Gyr; if foreground RGB stars contribute to the luminous tail, then the derived age for the stars is older still. The luminosity function also suggests that most of the intracluster stars are moderately metal-rich (-0.8 < [Fe/H] <-0.2), a result consistent with that expected from stars that have been tidally stripped from intermediate luminosity galaxies. Additionally, a comparison with the planetary nebulae in our field also supports this view, although the existence of a more metal-poor population (from stripped dwarfs) cannot be ruled out. Our derived average surface brightness, mu_I = 27.9 +0.3/-0.5 mag/arcsec^2 for Virgo's diffuse component suggests that intracluster stars contribute 10% to 20% of the cluster's total I-band luminosity.Comment: 21 pages, 8 figures included, accepted for publication in the Astrophysical Journa

The optical/infrared counterpart(s) of IRAS 18333-2357

Observations of the potential optical counterparts of the unusual source IRAS 18333-2357 show that this source is associated with an extraordinary planetary nebula system in the galactic globular cluster M22. Three distinct optical objects were found within 2" of the IRAS 18333-2357 position as determined by precisely locating the 20 μm infrared source. One object is a red star with m_v ≈ 14.7 mag, which appears to be an unrelated background field star that is possibly significantly reddened beyond the line-of-sight reddening to M22. The second stellar object is a very blue star with m_v ≈ 14.3 mag located about 1~3 south of the red star. Absorption lines of He 11 and possibly H are present in 4000-5000 A spectra of the stellar pair, similar to spectra of planetary nebula nuclei. The third member of this optical triple is an extended emission line nebulosity approximately 10" x 7" in size, centered about 1" east and south of the red star. The ionized gas in this nebulosity is extraordinarily oxygen-rich and neon-rich relative to both hydrogen and helium compared to the atmospheres of M22 red giants and is substantially oxygen-rich and neon-rich relative to hydrogen in comparison with typical planetary nebulae. This nebulosity is almost certainly in M22. The blue star is also very likely to be a member of M22, the source of ionizing photons for the nebulosity and probably the luminosity source for IRAS 18333-2357. We suggest that the dust responsible for the strong infrared emission of IRAS 18333-2357 is physically associated with the M22 nebulosity. In this case the total nebular mass, comprised of 3-10 x 10^(-4) M_☉ of ionized gas plus > 6 x 10^(-4) M_☉ of silicate or carbon-based grains, is possibly dominated by the dust component. The 0, Mg, Si abundances in the case of silicate grains, or carbon in the case of carbon-based grains, may be enhanced relative to hydrogen by at least a factor of 1000 compared to solar abundances. The relative abundances and mass of the M22 nebula are very unusual among known planetary nebulae. It is speculated that this system in M22 may be the result of the interaction within a close binary system containing at least one 0-Ne white dwarf component, or perhaps related to planetary nebulae like A30 and A78