R. W. Evans

California Institute of Technology, Pasadena, California, United States

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Publications (12)49.08 Total impact

  • Minor Planet Circulars. 06/2003; 48624:2.
  • Icarus 02/1998; 131:261-282. · 3.16 Impact Factor
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    ABSTRACT: Using the Hubble Space Telescope (HST) and Wide Field Planetary Camera 2, we have imaged the central 20 pc of the giant H ii region 30 Doradus Nebula in three different emission lines. The images allow us to study the nebula with a physical resolution that is within a factor of 2 of that of typical ground-based observations of Galactic H ii regions. We present a gallery of interesting objects within the region studied. These include a tube blown by the wind of a high-velocity star and a discrete H ii region around an isolated B star. This small isolated H ii region appears to be in the midst of the champagne flow phase of its evolution. Most of the emission within 30 Dor is confined to a thin zone located between the hot interior of the nebula and surrounding dense molecular material. This zone appears to be directly analogous to the photoionized photoevaporative flows that dominate emission from small, nearby H ii regions. For example, a column of material protruding from the cavity wall to the south of the main cluster is found to be a direct analog to elephant trunks in M16. Surface brightness profiles across this structure are very similar to surface brightness profiles taken at ground-based resolution across the head of the largest column in M16. The dynamical effects of the photoevaporative flow can be seen as well. An arcuate feature located above this column and a similar feature surrounding a second nearby column are interpreted as shocks in which the photoevaporative flow stagnates against the high-temperature gas that fills the majority of the nebula. The ram pressure in the photoevaporative flow, derived from thermal pressure at the surface of the column, is found to balance with the pressure in the interior of the nebula derived from previous X-ray observations. By analogy with the comparison of ground-based and HST images of M16, we infer that the same sharply stratified structure seen in HST images of M16 almost certainly underlies the observed structure in 30 Doradus, which is a crucial case because it allows us to bridge the gap between nearby H ii regions and the giant H ii regions seen in distant galaxies. The real significance of this result is that it demonstrates that the physical understanding gained from detailed study of photoevaporative interfaces in nearby H ii regions can be applied directly to interpretation of giant H ii regions. Stated another way, interpretation of observations of giant H ii regions must account for the fact that this emission arises not from expansive volumes of ionized gas but instead from highly localized and extremely sharply stratified physical structures.
    The Astronomical Journal 01/1998; 116:163-179. · 4.97 Impact Factor
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    ABSTRACT: We present deep photometry obtained with the HST in an outer LMC field. A well-defined main sequence is seen down to V>26. We derive a luminosity function from the data and use it to constrain the IMF and the star formation history. We derive limits on the IMF slope, alpha (with dN/dM~ M(alpha ) ), from stars on the main sequence which are fainter than the oldest turnoff. For most choices of star formation history and metallicity, we derive slopes which are consistent the the Salpeter (alpha = -2.35) or local solar neighborhood IMF, although the preferred values are steeper. We can rule out IMF slopes shallower than -1.6 and steeper than -3.1 for the mass range 0.6 <~ M <~ 1.1 \Msun. Assuming a Salpeter IMF over the entire observed mass range, we derive star formation histories from the entire luminosity function, which covers the mass range 0.6 <~ M <~ 3 \Msun. We find that the luminosity function is inconsistent with the scenario in which the bulk of the field stars in the LMC are younger than 4 Gyr. Instead, we find that there must be a comparable number of stars older and younger than 4 Gyr. Our best model has a star formation rate which is roughly constant for 10 Gyr then increases by about a factor of three for the past 2 Gyr. Such a model is also roughly consistent with the distribution of stars in the color-magnitude diagram. Similar model parameters are derived if we adopt the Kroupa, Tout, and Gilmore solar neighborhood IMF instead of a Salpeter slope. Alternatively, we can fit the luminosity function with a predominantly young population if we use a steeper single power law IMF slope with alpha ~ -2.75 over the entire range of observed masses.
    The Astronomical Journal 01/1997; 113:656-668. · 4.97 Impact Factor
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    ABSTRACT: Two interacting galaxies and four central galaxies in cooling flow clusters were observed with the Hubble Space Telescope to look for young compact massive star clusters. The two interacting galaxies, NGC 3597 and NGC 6052, show clear evidence for resolved bright blue clusters that are likely to be comparable in mass to Galactic globulars based on their colors and brightnesses. In NGC 3597, the clusters have physical sizes comparable to Galactic globulars. In NGC 6052, there are a few compact clusters, but most appear more extended. No such objects were seen in three of the four cooling flow galaxies. The central galaxy in Abell 1795 may have several massive blue clusters, but its distance prevents us from knowing how compact they are.
    The Astronomical Journal 07/1996; 112:416. · 4.97 Impact Factor
  • R. Sahai, J. T. Trauger, R. W. Evans, Idt
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    ABSTRACT: As part of a multi-wavelength study of the mass-loss processes which red giant stars undergo on the Asymptotic Giant Branch (AGB) and beyond, we have obtained high spatial-resolution images of two post-AGB objects using the WFPC2 aboard the HST. The first is CRL2688 (the Egg Nebula), belonging to a class of bipolar reflection nebulae surrounding post-AGB stars in transition to becoming planetary nebulae. Our F606W (wide-band 606 nm filter) image shows a dark edge-on flaring disk obscuring the central star, a pair of radial "searchlight-beam" like features, criss-crossed by a large number (at least 25) of roughly circular arcs around the center. The arcs probably represent local peaks in a quasi-periodic mass-ejection process. Intriguingly, some of the arcs clearly intersect and/or bifurcate. Very faint radial striations can be seen within the "searchlight-beam" structures, indicating that these are jets of matter. We have also used the WFPC2 POLQ polarisation-quad filter to image the polarisation characteristics of the Egg Nebula. The second object is the young planetary nebula, MyCn-18, selected because of its striking resemblance to the triple-ring nebula around SN1987A (Burrows et al. 1995, ApJ, in press). We have imaged MyCn-18 in several narrow emission-line filters. Our Hα and [NII] images show an hourglass-shaped nebula, with the outer rings representing the hourglass rims. A pair of intersecting elliptical rings in the central region may be the rims of a smaller hourglass. A series of arcs appear to be engraved on the hourglass walls. Within the inner hourglass, one can see a barrel-shaped structure whose symmetry axis is apparently orthogonal to the axis of the hourglasses. We will characterise the detailed structure of the circumstellar material in CRL2688 and MyCn-18 which has been revealed unambiguously for the first time in our WFPC2 images, and discuss possible mechanisms for producing such structures, with reference to current theories for the formation of planetary nebulae.
    11/1995; 27:1344.
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    01/1995; -1:379.
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    ABSTRACT: We present UVRI and Halpha images of the nuclear starburst in NGC 253, a barred spiral galaxy located at a distance of only 3 Mpc. We resolve the nuclear region into many star-forming complexes, most of which are themselves resolved for the first time and have typical diameters of up to about 5 pc. We also resolve the brightest stars in the circumnuclear region. The Halpha emission lies in a fan to the SE of the nucleus, and both it and dust absorption has a distinctive radial filamentary structure. This is probably the result of the superwind being driven by the starburst.
    12/1994;
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    ABSTRACT: We have observed HH 30 using the WFPC2 Wide Field Camera aboard the Hubble Space Telescope. We find three major components to this object: 1) A shield-shaped reflection nebula 2.5('') long, 0.5('') wide that extends from SE to NW; 2) A highly collimated optical jet emerging perpendicular to the reflection nebula; and 3) a second fainter reflection nebula SW of the shield nebula, and separated from it by 0.5('') . The observed morphology is strikingly similar to models of a optically thick circumstellar disk viewed from near the equator plane, with a jet emerging along the disk rotation axis. With a diameter of 30 AU, the HH 30 jet is significantly narrower than other Herbig-Haro jets whose widths have been resolved. We present models for the density structure which produces the reflection nebulosities, and find that models with an obscured central star, disk diameter of at least 300 AU, and steep flaring at distances > 50 AU provide a good description of the object.
    12/1994;
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    ABSTRACT: Neutral sodium emissions encircling Jupiter exhibit an intricate and variable structure that is well matched by a simple loss process from Io's atmosphere. These observations imply that fast neutral sodium is created locally in the Io plasma torus, both near Io and as much as 8 hours downstream. Sodium-bearing molecules may be present in Io's upper atmosphere, where they are ionized by the plasma torus and swept downstream. The molecular ions dissociate and dissociatively recombine on a short time scale, releasing neutral fragments into escape trajectories from Jupiter. This theory explains a diverse set of sodium observations, and it implies that molecular reactions (particularly electron impact ionization and dissociation) are important at the top of Io's atmosphere.
    Science 10/1991; 253(5026):1394-7. · 31.03 Impact Factor