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Caleb A. Scharf

Publications and source records attributed to Caleb A. Scharf.

15 recordsLinked to original sources
arXiv2010

Habitable Climates: The Influence of Eccentricity

In the outer regions of the habitable zone, the risk of transitioning into a globally frozen "snowball" state poses a threat to the habitability of planets with the capacity to host water-based life. We use a one-dimensional energy balance climate model (EBM) to examine how obliquity, spin rate, orbital eccentricity, and ocean coverage might influence the onset of such a snowball state. For an exoplanet, these parameters may be strikingly different from the values observed for Earth. Since, for constant semimajor axis, the annual mean stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest habitable semimajor axis (for fixed atmospheric composition) to scale as (1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower bound on the outer boundary of the habitable zone, but the influence of obliquity and ocean fraction can be profound in the context of planets on eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending on obliquity, with higher obliquity worlds generally more stable against snowball transitions. One might also expect that the long winter at an eccentric planet's apoastron would render it more susceptible to global freezing. Our models suggest that this is not a significant risk for Earth-like planets around Sun-like stars since such planets are buffered by the thermal inertia provided by oceans covering at least 10% of their surface. Since planets on eccentric orbits spend much of their year particularly far from the star, such worlds might turn out to be especially good targets for direct observations with missions such as TPF-Darwin. Nevertheless, the extreme temperature variations achieved on highly eccentric exo-Earths raise questions about the adaptability of life to marginally or transiently habitable conditions.

astro-ph.EP↗
arXiv2010

Generalized Milankovitch Cycles and Longterm Climatic Habitability

Although the Earth's orbit is never far from circular, terrestrial planets around other stars might experience substantial changes in eccentricity that could lead to climate changes, including possible "phase transitions" such as the snowball transition (or its opposite). There is evidence that Earth has gone through at least one globally frozen, "snowball" state in the last billion years, which it is thought to have exited after several million years because global ice-cover shut off the carbonate-silicate cycle, thereby allowing greenhouse gases to build up to sufficient concentration to melt the ice. Due to the positive feedback caused by the high albedo of snow and ice, susceptibility to falling into snowball states might be a generic feature of water-rich planets with the capacity to host life. This paper has two main thrusts. First, we revisit one-dimensional energy balance climate models as tools for probing possible climates of exoplanets, investigate the dimensional scaling of such models, and introduce a simple algorithm to treat the melting of the ice layer on a globally-frozen planet. We show that if a terrestrial planet undergoes Milankovitch-like oscillations of eccentricity that are of great enough magnitude, it could melt out of a snowball state. Second, we examine the kinds of variations of eccentricity that a terrestrial planet might experience due to the gravitational influence of a giant companion. We show that a giant planet on a sufficiently eccentric orbit can excite extreme eccentricity oscillations in the orbit of a habitable terrestrial planet. More generally, these two results demonstrate that the longterm habitability (and astronomical observables) of a terrestrial planet can depend on the detailed architecture of the planetary system in which it resides.

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arXiv2010

Possible constraints on exoplanet magnetic field strengths from planet-star interaction

A small percentage of normal stars harbor giant planets that orbit within a few tenths of an astronomical unit. At such distances the potential exists for significant tidal and magnetic field interaction resulting in energy dissipation that may manifest as changes within the stellar corona. We examine the X-ray emission of stars hosting planets and find a positive correlation between X-ray luminosity and the projected mass of the most closely orbiting exoplanets. We investigate possible systematics and observational biases that could mimic or confuse this correlation but find no strong evidence for any, especially for planets more massive than ~0.1 MJ. Luminosities and upper limits are consistent with the interpretation that there is a lower floor to stellar X-ray emission dependent on close-in planetary mass. Under the hypothesis that this is a consequence of planet-star magnetic field interaction, and energy dissipation, we estimate a possible field strength increase between planets of 1 and 10 MJ of a factor ~8. Intriguingly, this is consistent with recent geodynamo scaling law predictions. The high-energy photon emission of planet-star systems may therefore provide unique access to the detailed magnetic, and hence geodynamic, properties of exoplanets.

astro-ph.SR↗
arXiv2009

New Worlds: Evaluating terrestrial planets as astrophysical objects

Terrestrial exoplanets are on the verge of joining the ranks of astronomically accessible objects. Interpreting their observable characteristics, and informing decisions on instrument design and use, will hinge on the ability to model these planets successfully across a vast range of configurations and climate forcings. A hierarchical approach that addresses fundamental behaviors as well as more complex, specific, situations is crucial to this endeavor and is presented here. Incorporating Earth-centric knowledge, and continued cross-disciplinary work will be critical, but ultimately the astrophysical study of terrestrial exoplanets must be encouraged to develop as its own field.

astro-ph.EP↗
arXiv2008

Habitable Climates: The Influence of Obliquity

Extrasolar terrestrial planets with the potential to host life might have large obliquities or be subject to strong obliquity variations. We revisit the habitability of oblique planets with an energy balance climate model (EBM) allowing for dynamical transitions to ice-covered snowball states as a result of ice-albedo feedback. Despite the great simplicity of our EBM, it captures reasonably well the seasonal cycle of global energetic fluxes at Earth's surface. It also performs satisfactorily against a full-physics climate model of a highly oblique Earth-like planet, in an unusual regime of circulation dominated by heat transport from the poles to the equator. Climates on oblique terrestrial planets can violate global radiative balance through much of their seasonal cycle, which limits the usefulness of simple radiative equilibrium arguments. High obliquity planets have severe climates, with large amplitude seasonal variations, but they are not necessarily more prone to global snowball transitions than low obliquity planets. We find that terrestrial planets with massive CO2 atmospheres, typically expected in the outer regions of habitable zones, can also be subject to such dynamical snowball transitions. Some of the snowball climates investigated for CO2-rich atmospheres experience partial atmospheric collapse. Since long-term CO2 atmospheric build-up acts as a climatic thermostat for habitable planets, partial CO2 collapse could limit the habitability of such planets. A terrestrial planet's habitability may thus depend sensitively on its short-term climatic stability.

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arXiv2008

Habitable Climates

According to the standard liquid-water definition, the Earth is only partially habitable. We reconsider planetary habitability in the framework of energy-balance models, the simplest seasonal models in physical climatology, to assess the spatial and temporal habitability of Earth-like planets. We quantify the degree of climatic habitability of our models with several metrics of fractional habitability. Previous evaluations of habitable zones may have omitted important climatic conditions by focusing on close Solar System analogies. For example, we find that model pseudo-Earths with different rotation rates or different land-ocean fractions have fractional habitabilities that differ significantly from that of the Earth itself. Furthermore, the stability of a planet's climate against albedo-feedback snowball events strongly impacts its habitability. Therefore, issues of climate dynamics may be central in assessing the habitability of discovered terrestrial exoplanets, especially if astronomical forcing conditions are different from the moderate Solar System cases.

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arXiv2007

A possible dearth of hot gas in galaxy groups at intermediate redshift

We examine the X-ray luminosity of galaxy groups in the CNOC2 survey, at redshifts 0.1 < z < 0.6. Previous work examining the gravitational lensing signal of the CNOC2 groups has shown that they are likely to be genuine, gravitationally bound objects. Of the 21 groups in the field of view of the EPIC-PN camera on XMM-Newton, not one was visible in over 100 ksec of observation, even though three of the them have velocity dispersions high enough that they would easily be visible if their luminosities scaled with their velocity dispersions in the same way as nearby groups' luminosities scale. We consider the possibility that this is due to the reported velocity dispersions being erroneously high, and conclude that this is unlikely. We therefore find tentative evidence that groups at intermediate redshift are underluminous relative to their local cousins.

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arXiv2007

Exoplanet Transit Parallax

The timing and duration of exoplanet transits has a dependency on observer position due to parallax. In the case of an Earth-bound observer with a 2 AU baseline the dependency is typically small and slightly beyond the limits of current timing precision capabilities. However, it can become an important systematic effect in high-precision repeated transit measurements for long period systems due to its relationship to secular perspective acceleration phenomena. In this short paper we evaluate the magnitude and characteristics of transit parallax in the case of exoplanets using simplified geometric examples. We also discuss further implications of the effect, including its possible exploitation to provide immediate confirmation of planetary transits and/or unique constraints on orbital parameters and orientations.

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arXiv2006

The potential for tidally heated icy and temperate moons around exoplanets

Moons of giant planets may represent an alternative to the classical picture of habitable worlds. They may exist within the circumstellar habitable zone of a parent star, and through tidal energy dissipation they may also offer alternative habitable zones, where stellar insolation plays a secondary, or complementary, role. We investigate the potential extent of stable satellite orbits around a set of 74 known extrasolar giant planets located beyond 0.6 AU from their parent stars - where moons should be long-lived with respect to removal by stellar tides. Approximately 60% of these giant planets can sustain satellites or moons in bands up to $\sim 0.04$ AU in width. For comparison, the Galiean satellites extend to $\sim 0.013$ AU. We investigate the stellar insolation that moons would experience for these exoplanet systems, and the implications for sublimation loss of volatiles. We find that between 15 and 27% of {\em all} known exoplanets may be capable of harboring small, icy, moons. In addition, some 22-28% of all known exoplanets could harbor moons within a ``sublimation zone'', with insolation temperatures between 273 K and 170 K. A simplified energy balance model is applied to the situation of temperate moons, maintained by a combination of stellar insolation and tidal heat flow. We demonstrate that large moons ($>0.1 $M$_{\oplus}$), at orbital radii commensurate with those of the Galilean satellites, could maintain temperate, or habitable, surface conditions during episodes of tidal heat dissipation of the order 1-100 times that currently seen on Io. (Abridged).

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arXiv2003

The Evolution of Cluster Radio Galaxies at 0.5<z<1

We are conducting a deep radio survey of a sample of 25 distant ($0.5<z<1$) clusters of galaxies. Here we present a progress report. So far 17 of 25 clusters have been observed, to varying depths. We have found 33 radio sources within 0.3 Abell radii of the cluster center, of which 28 are likely associated with cluster member galaxies. A comparison of the radio luminosity function of these clusters with the results of two surveys at lower redshift, reveals no evidence for strong evolution in the population of cluster radio galaxies at $z \sim 0.65$.

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arXiv2002

A statistical detection of gamma-ray emission from galaxy clusters: implications for the gamma-ray background and structure formation

The origin of the diffuse extragalactic, high-energy gamma-ray background (EGRB) filling the Universe remains unknown. The spectrum of this extragalactic radiation, as measured by the EGRET on-board CGRO, is well-fit by a power law across nearly four decades in energy, from 30 MeV to 100 GeV. It has been estimated that not more than a quarter of the diffuse gamma-ray background could be due to unresolved point sources. Recent studies have suggested that much of the diffuse background could originate from the up-scatter of CMB photons by relativistic electrons produced by shock waves in the IGM during large-scale structure formation. In this work we search for evidence of gamma-ray emission associated with galaxy clusters by cross-correlating high Galactic latitude EGRET data with Abell clusters. Our results indicate a possible association of emission with clusters at a >3-sigma level. For a subset of the 447 richest (R>=2) clusters the mean surface brightness excess is 1.2x10^-6 ph cm^-2 s^-1 sr^-1 (>100MeV), corresponding to a typical non-thermal bolometric luminosity of L_{gamma} ~ 1x10^44 erg s^-1. Extrapolating this measurement and assuming no evolution we conservatively estimate that ~ 1-10 of the EGRB could originate from clusters with z<1. For this cluster population the predicted non-thermal luminosity is in excellent agreement with our measurement, suggesting that the clusters have experienced mass accretion within the last 10^9 yrs. If correct, then future gamma-ray missions, such as GLAST should be able to directly detect nearby galaxy clusters.

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arXiv2002

Optimal Chandra/XMM-Newton Band-Passes for Detecting Low Temperature Groups and Clusters of Galaxies

In this short paper I present the results of a calculation which seeks the maximum, or optimal, signal-to-noise energy band for galaxy group or cluster X-ray emission detected by the Chandra and XMM-Newton observatories. Using a background spectrum derived from observations and a grid of models I show that the "classical" 0.5-2 keV band is indeed close to optimal for clusters with gas temperatures >2 keV, and redshifts z<1. For cooler systems however, this band is generally far from optimal. Sub-keV plasmas can suffer 20-60% signal-to-noise loss compared to an optimal band, and worse for z>0. The implication is that current and forthcoming surveys should be carefully constructed in order to minimize bias against the low mass, low temperature end of the cluster/group population.

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arXiv1999

The Second Most Distant Cluster of Galaxies in the EMSS

We report on our ASCA, Keck, and ROSAT observations of MS1137.5+6625, the second most distant cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), at redshift 0.78. We now have X-ray temperatures, velocity dispersions, and X-ray images for a complete, high-redshift sample of EMSS clusters. ASCA observations of MS1137.5+6625 yield a temperature of 5.7 +2.1, -1.1 keV and a metallicity of 0.43 +0.40,-0.37 solar (90% confidence). Keck-II spectroscopy of 22 cluster members reveals a velocity dispersion of 884 +185, -124 km/s. ROSAT observations show that MS1137.5+6625 is regular and centrally concentrated. A beta-model fit yields a core radius of only 71 h^{-1} kpc (q_0=0.1) with beta=0.70 +0.45,-0.15. The gas fraction is 0.06 +/-0.04 h^{-3/2}. This is the highest redshift EMSS cluster with a possible cooling flow (~20-400 solar masses/yr). The velocity dispersion, temperature, gas fraction, and iron abundance of MS1137 are statistically the same as those properties in low-z clusters of similar luminosity. We derive a high-z temperature function for EMSS clusters with 0.5 < z < 0.9 and compare it with temperature functions at lower redshifts, showing that evolution of the temperature function is relatively modest. Supplementing our high-z sample with other data, we demonstrate that neither the cluster L-T relation, nor cluster metallicities, nor the cluster gas fraction has detectably evolved with redshift. The very modest degree of evolution in the L-T relation inferred from these data is inconsistent with the absence of evolution in the X-ray luminosity functions derived from ROSAT cluster surveys if a critical-density structure formation model is assumed.

astro-ph↗
arXiv1999

Observational Tests of the Mass-Temperature Relation for Galaxy Clusters

We examine the relationship between the mass and x-ray gas temperature of galaxy clusters using data drawn from the literature. Simple theoretical arguments suggest that the mass of a cluster is related to the x-ray temperature as $M \propto T_x^{3/2}$. Virial theorem mass estimates based on cluster galaxy velocity dispersions seem to be accurately described by this scaling with a normalization consistent with that predicted by the simulations of Evrard, Metzler, & Navarro (1996). X-ray mass estimates which employ spatially resolved temperature profiles also follow a $T_x^{3/2}$ scaling although with a normalization about 40% lower than that of the fit to the virial masses. However, the isothermal $β$-model and x-ray surface brightness deprojection masses follow a steeper $\propto T_x^{1.8-2.0}$ scaling. The steepness of the isothermal estimates is due to their implicitly assumed dark matter density profile of $ρ(r) \propto r^{-2}$ at large radii while observations and simulations suggest that clusters follow steeper profiles (e.g., $ρ(r) \propto r^{-2.4}$).

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