Misty Cracraft

PROBING FOR EVIDENCE OF PLUMES ON EUROPA WITH HST/STIS

Roth et al (2014a) reported evidence for plumes of water venting from a southern high latitude region on Europa - spectroscopic detection of off-limb line emission from the dissociation products of water. Here, we present Hubble Space Telescope (HST) direct images of Europa in the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which plume activity could be implicated. Two show statistically significant features at latitudes similar to Roth et al, and the third, at a more equatorial location. We consider potential systematic effects that might influence the statistical analysis and create artifacts, and are unable to find any that can definitively explain the features, although there are reasons to be cautious. If the apparent absorption features are real, the magnitude of implied outgassing is similar to that of the Roth et al feature, however the apparent activity appears more frequently in our data.

V838 MONOCEROTIS: A GEOMETRIC DISTANCE FROM HUBBLE SPACE TELESCOPE POLARIMETRIC IMAGING OF ITS LIGHT ECHO*

Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct geometric distance determination, because it should contain a ring of highly linearly polarized light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys on board the Hubble Space Telescope, which confirms the presence of the highly polarized ring. Based on detailed modeling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarization, we find a distance of 6.1 ± 0.6 kpc. The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization, taken to be θmax = 90° ± 5°. The polarimetric distance agrees remarkably well with a distance of 6.2 ± 1.2 kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838 Mon. At this distance, V838 Mon at maximum light had MV −9.8, making it temporarily one of the most luminous stars in the Local Group. Our validation of the polarimetric method offers promise for measurement of extragalactic distances using supernova light echoes.

Active Cryovolcanism on Europa

Evidence for plumes of water on Europa has previously been found using the Hubble Space Telescope (HST) using two different observing techniques. Roth et al. (2014) found line emission from the dissociation products of water. Sparks et al. (2016) found evidence for off-limb continuum absorption as Europa transited Jupiter. Here, we present a new transit observation of Europa that shows a second event at the same location as a previous plume candidate from Sparks et al. (2016), raising the possibility of a consistently active source of erupting material on Europa. This conclusion is bolstered by comparison with a nighttime thermal image from the Galileo Photopolarimeter-Radiometer (PPR) which shows a thermal anomaly at the same location, within the uncertainties (Spencer et al. 1999). The anomaly has the highest observed brightness temperature on the Europa nightside. If heat flow from a subsurface liquid water reservoir causes the thermal anomaly, its depth is ~1.8-2 km, under simple modeling assumptions, consistent with scenarios in which a liquid water reservoir has formed within a thick ice shell. Models that favor thin regions within the ice shell that connect directly to the ocean, however, cannot be excluded, nor modifications to surface thermal inertia by subsurface activity. Alternatively, vapor deposition surrounding an active vent could increase the thermal inertia of the surface and cause the thermal anomaly. This candidate plume region may offer a promising location for an initial characterization of Europas internal water and ice and for seeking evidence of Europas habitability. ~

HUNDRED THOUSAND DEGREE GAS IN THE VIRGO CLUSTER OF GALAXIES

The physical relationship between low-excitation gas filaments at ~104 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~107 K in the centers of many galaxy clusters is not understood. It is unclear whether the ~104 K filaments have cooled and condensed from the ambient hot (~107 K) medium or have some other origin such as the infall of cold gas in a merger, or the disturbance of an internal cool reservoir of gas by nuclear activity. Observations of gas at intermediate temperatures (~105-106 K) can potentially reveal whether the central massive galaxies are gaining cool gas through condensation or losing it through conductive evaporation and hence identify plausible scenarios for transport processes in galaxy cluster gas. Here we present spectroscopic detection of ~105 K gas spatially associated with the Hα filaments in a central cluster galaxy, M87, in the Virgo Cluster. The measured emission-line fluxes from triply ionized carbon (C IV 1549 A) and singly ionized helium (He II 1640 A) are consistent with a model in which thermal conduction determines the interaction between hot and cold phases.

Discovery of C IV Emission Filaments in M87

ABSTRACTGas at intermediate temperatures between the hot X-ray-emitting coronal gas in galaxies at the centers of galaxyclusters and the much cooler optical line emitting filaments yields information on transport processes and plausiblescenarios for the relationship between X-ray cool cores and other galactic phenomena such as mergers or the onsetof an active galactic nucleus. Hitherto, detection of intermediate temperature gas has proven elusive. Here, wepresent FUV imaging of the “low excitation” emission filaments of M87 and show strong evidence for the presenceof Civ 1549 A emission which arises in gas at temperature ∼10 5 K co-located with H α +[Nii] emission fromcooler ∼10 4 K gas. We infer that the hot and cool phases are in thermal communication, and show that quanti-tatively the emission strength is consistent with thermal conduction, which in turn may account for many of theobserved characteristics of cool-core galaxy clusters. Key words: conduction – galaxies: individual (M87) – galaxies: ISM1. INTRODUCTIONThe close correlation between “cool-core clusters” viewedin X-rays, and optical emission-line nebulae in clusters ofgalaxies has been recognized for many years, but the physicalreason for this connection remains unclear. Optical emission-line filamentary structures have been seen and analyzed inmany cool-core clusters (e.g., Sparks et al. 1989, 2004;Voit& Donahue 1997;Crawfordetal.2005; Conselice et al.2001; Hatch et al. 2006, and references therein). Suggestedformation scenarios for the “optical” filaments have included(1) condensations in cooling intracluster medium (e.g., FabianN Cowie et al. 1980; Fabian et al. 1984; Heckmanetal.1989;DonahueV Revaz et al. 2007),(2)alreadycold interstellar material originating in galaxies falling into thecluster core (e.g., Rubin et al. 1977; Sparks et al. 1989; Sparks1992; Braine et al. 1995; Yagi et al. 2007), and (3) already coldmaterial originating in the central cluster galaxy (e.g., Fabianet al. 2003;Crawfordetal.2005).Many different excitation mechanisms for the filaments havebeen considered. For NGC 1275 (Sabra et al. 2000; Conseliceet al. 2001) and M87 (Sabra et al. 2003) photoionizationby the central active galactic nucleus (AGN) and by theintracluster medium, by shocks, and by hot, young stars were allconsidered. These authors concluded that “neither shocks norphotoionization alone can reproduce the emission-line intensityratios” and that some additional source of heating must bepresent. A study of the optical line ratios in Abell 2597 ledVoit & Donahue (1997) to rule out shocks as an excitationmechanism, and to conclude that although hot stars might bethe best candidate for producing the ionization, even the hotteststars could not power a nebula as hot as the one observed andthat some other non-ionizing source of heat must contribute acomparable amount of power. Similarconclusions were reachedby Hatch et al. (2007) for a number of cool-core clusters.One obvious source of such extra heating comes from the factthat the relatively cool H

On the nature of optical nuclei in FR I radio-galaxies from ACS/HST imaging polarimetry ⋆

We obtained optical imaging polarimetry with the ACS/HRC aboard the HST of the 9 closest radio- galaxies in the 3C catalogue with an FR I morphology. The nuclear sources seen in direct HST images in these galaxies are found to be highly polarized with levels in the range � 2 - 11 % with a median value of 7 %. We discuss the different mechanisms that produce polarized emission and conclude that the only viable interpretation is a synchrotron origin for the optical nuclei. This idea is strengthened by the analogy with the polarization properties of BL Lac objects, providing also further support to the FR I/BL Lac unified model. This confirms previous suggestions that the dominant emission mechanism in low luminosity radio-loud AGN is related to non-thermal radiation produced by the base of their jets. In addition to the nuclear polarization (and to the large scale optical jets), polarization is detected co-spatially with the dusty circumnuclear disks, likely due to dichroic transmission; the polarization vectors are tangential to the disks as expected when the magnetic field responsible for the grains alignment is stretched by differential rotation. We explored the possibility to detect the polarimetric signature of a misaligned radiation beam in FR I, expected in our sources in the frame of the FR I/ BL Lac unification. We did not find this effect in any of the galaxies, but our the results are not conclusive on whether a misaligned beam is indeed present in FR I.

Interstellar-medium mapping in M82 through light echoes around supernova 2014J

We present multiple-epoch measurements of the size and surface brightness of the light echoes from supernova (SN) 2014J in the nearby starburst galaxy M82. Hubble Space Telescope (HST) ACS/WFC images were taken ~277 and ~416 days after B-band maximum in the filters F475W, F606W, and F775W. Observations with HST WFC3/UVIS images at epochs ~216 and ~365 days (Crotts 2015) are included for a more complete analysis. The images reveal the temporal evolution of at least two major light-echo components. The first one exhibits a filled ring structure with position-angle-dependent intensity. This radially extended, diffuse echo indicates the presence of an inhomogeneous interstellar dust cloud ranging from ~100 pc to ~500 pc in the foreground of the SN. The second echo component appears as an unresolved luminous quarter-circle arc centered on the SN. The wavelength dependence of scattering measured in different dust components suggests that the dust producing the luminous arc favors smaller grain sizes, while that causing the diffuse light echo may have sizes similar to those of the Milky Way dust. Smaller grains can produce an optical depth consistent with that along the supernova-Earth line of sight measured by previous studies around maximum light. Therefore, it is possible that the dust slab, from which the luminous arc arises, is also responsible for most of the extinction towards SN 2014J. The optical depths determined from the Milky Way-like dust in the scattering matters are lower than that produced by the dust slab.

The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System

We describe the layout and unique features of the focal plane system for MIRI. We begin with the detector array and its readout integrated circuit (combining the amplifier unit cells and the multiplexer), the electronics, and the steps by which the data collection is controlled and the output signals are digitized and delivered to the JWST spacecraft electronics system. We then discuss the operation of this MIRI data system, including detector readout patterns, operation of subarrays, and data formats. Finally, we summarize the performance of the system, including remaining anomalies that need to be corrected in the data pipeline.

The long-period Galactic Cepheid RS Puppis - III. A geometric distance from HST polarimetric imaging of its light echoes

As one of the most luminous Cepheids in the Milky Way, the 41.5-day RS Puppis is an analog of the long-period Cepheids used to measure extragalactic distances. An accurate distance to this star would therefore help anchor the zero-point of the bright end of the period-luminosity relation. But, at a distance of about 2 kpc, RS Pup is too far away for measuring a direct trigonometric parallax with a precision of a few percent with existing instrumentation. RS Pup is unique in being surrounded by a reflection nebula, whose brightness varies as pulses of light from the Cepheid propagate outwards. We present new polarimetric imaging of the nebula obtained with HST/ACS. The derived map of the degree of linear polarization pL allows us to reconstruct the three-dimensional structure of the dust distribution. To retrieve the scattering angle from the pL value, we consider two different polarization models, one based on a Milky Way dust mixture and one assuming Rayleigh scattering. Considering the derived dust distribution in the nebula, we adjust a model of the phase lag of the photometric variations over selected nebular features to retrieve the distance of RS Pup. We obtain a distance of 1910 +/- 80 pc (4.2%), corresponding to a parallax of 0.524 +/- 0.022 mas. The agreement between the two polarization models we considered is good, but the final uncertainty is dominated by systematics in the adopted model parameters. The distance we obtain is consistent with existing measurements from the literature, but light echoes provide a distance estimate that is not subject to the same systematic uncertainties as other estimators (e.g. the Baade-Wesselink technique). RS Pup therefore provides an important fiducial for the calibration of systematic uncertainties of the long-period Cepheid distance scale.

The Mid-Infrared Instrument for the James Webb Space Telescope, X. Operations and Data Reduction

We describe the operations concept and data reduction plan for the Mid- Infrared Instrument (MIRI) for the James Webb Space Telescope (JWST). The overall JWST operations concept is to use Observation Templates (OTs) to provide a straightforward and intuitive way for users to specify observations. MIRI has four OTs that correspond to the four observing modes: 1.) Imaging, 2.) Coronagraphy, 3.) Low Resolution Spectroscopy, and 4.) Medium Resolution Spectroscopy. We outline the user choices and expansion of these choices into detailed instrument operations. The data reduction plans for MIRI are split into three stages, where the specificity of the reduction steps to the observation type increases with stage. The reduction starts with integration ramps: stage 1 yields uncalibrated slope images; stage 2 calibrates the slope images; and then stage 3 combines multiple calibrated slope images into high level data products (e.g. mosaics, spectral cubes, and extracted source information). Finally, we give examples of the data and data products that will be derived from each of the four different OTs.