Gaseous Debris Research Articles

Fast and Slow Precession of Gaseous Debris Disks around Planet-accreting White Dwarfs

Abstract Spectroscopic observations of some metal-rich white dwarfs (WDs), believed to be polluted by planetary material, reveal the presence of compact gaseous metallic disks orbiting them. The observed variability of asymmetric, double-peaked emission-line profiles in about half of such systems could be interpreted as the signature of precession of an eccentric gaseous debris disk. The variability timescales—from decades down to 1.4 year (recently inferred for the debris disk around HE 1349–2305)—are in rough agreement with the rate of general relativistic (GR) precession in the test-particle limit. However, it has not been demonstrated that this mechanism can drive such a fast, coherent precession of a radially extended (out to 1 R ⊙ ) gaseous disk mediated by internal stresses (pressure). Here, we use the linear theory of eccentricity evolution in hydrodynamic disks to determine several key properties of eccentric modes in gaseous debris disks around WDs. We find a critical dependence of both the precession period and radial eccentricity distribution of the modes on the inner disk radius, r in. For small inner radii, r in ≲ ( 0.2 – 0.4 ) R ⊙ , the modes are GR-driven, with periods of ≈1–10 year. For r in ≳ ( 0.2 – 0.4 ) R ⊙ , the modes are pressure dominated, with periods of ≈3–20 year. Correspondence between the variability periods and inferred inner radii of the observed disks is in general agreement with this trend. In particular, the short period of HE 1349–2305 is consistent with its small r in. Circum-WD debris disks may thus serve as natural laboratories for studying the evolution of eccentric gaseous disks.

Rapid Evolution of the Gaseous Exoplanetary Debris around the White Dwarf Star HE 1349–2305

Abstract Observations of heavy metal pollution in white dwarf stars indicate that metal-rich planetesimals are frequently scattered into star-grazing orbits, tidally disrupted, and accreted onto the white dwarf surface, offering direct insight into the dynamical evolution of post-main-sequence exoplanetary systems. Emission lines from the gaseous debris in the accretion disks of some of these systems show variations on timescales of decades, and have been interpreted as the general relativistic precession of a recently formed, elliptical disk. Here we present a comprehensive spectroscopic monitoring campaign of the calcium infrared triplet emission in one system, HE 1349–2305, which shows morphological emission profile variations suggestive of a precessing, asymmetric intensity pattern. The emission profiles are shown to vary on a timescale of one to two years, which is an order of magnitude shorter than what has been observed in other similar systems. We demonstrate that this timescale is likely incompatible with general relativistic precession, and consider alternative explanations for the rapid evolution, including the propagation of density waves within the gaseous debris. We conclude with recommendations for follow-up observations, and discuss how the rapid evolution of the gaseous debris in HE 1349–2305 could be leveraged to test theories of exoplanetary debris disk evolution around white dwarf stars.

Evidence for Eccentric, Precessing Gaseous Debris in the Circumstellar Absorption toward WD 1145 + 017

Abstract We present time-series spectra revealing changes in the circumstellar line profiles for the white dwarf WD 1145 + 017. Over the course of 2.2 years, the spectra show complete velocity reversals in the circumstellar absorption, moving from strongly redshifted in 2015 April to strongly blueshifted in 2017 June. The depth of the absorption also varies, increasing by a factor of two over the same period. The dramatic changes in the line profiles are consistent with eccentric circumstellar gas rings undergoing general relativistic precession. As the argument of periapsis of the rings changes relative to the line of sight, the transiting gas shifts from receding in 2016 to approaching in 2017. Based on the precession timescales in the favored model, we make predictions for the line profiles over the next few years. Spectroscopic monitoring of WD 1145 + 017 will test these projections and aid in developing more accurate white dwarf accretion disk models.

WIRED for EC: New White Dwarfs with WISE Infrared Excesses and New Classification Schemes from the Edinburgh–Cape Blue Object Survey

Abstract We present a simple method for identifying candidate white dwarf systems with dusty exoplanetary debris based on a single temperature blackbody model fit to the infrared excess. We apply this technique to a sample of Southern Hemisphere white dwarfs from the recently completed Edinburgh–Cape Blue Object Survey and identify four new promising dusty debris disk candidates. We demonstrate the efficacy of our selection method by recovering three of the four Spitzer confirmed dusty debris disk systems in our sample. Further investigation using archival high-resolution imaging shows that Spitzer data of the unrecovered fourth object is likely contaminated by a line-of-sight object that either led to a misclassification as a dusty disk in the literature or is confounding our method. Finally, in our diagnostic plot, we show that dusty white dwarfs, which also host gaseous debris, lie along a boundary of our dusty debris disk region, providing clues to the origin and evolution of these especially interesting systems.

Detection of Submillimeter-wave [C i] Emission in Gaseous Debris Disks of 49 Ceti and β Pictoris

Abstract We have detected [C i] 3 P 1–3 P 0 emissions in the gaseous debris disks of 49 Ceti and β Pictoris with the 10 m telescope of the Atacama Submillimeter Telescope Experiment, which is the first detection of such emissions. The line profiles of [C i] are found to resemble those of CO(J = 3–2) observed with the same telescope and the Atacama Large Millimeter/submillimeter Array. This result suggests that atomic carbon (C) coexists with CO in the debris disks and is likely formed by the photodissociation of CO. Assuming an optically thin [C i] emission with the excitation temperature ranging from 30 to 100 K, the column density of C is evaluated to be (2.2 ± 0.2) × 1017 and (2.5 ± 0.7) × 1016 cm−2 for 49 Ceti and β Pictoris, respectively. The C/CO column density ratio is thus derived to be 54 ± 19 and 69 ± 42 for 49 Ceti and β Pictoris, respectively. These ratios are higher than those of molecular clouds and diffuse clouds by an order of magnitude. The unusually high ratios of C to CO are likely attributed to a lack of H2 molecules needed to reproduce CO molecules efficiently from C. This result implies a small number of H2 molecules in the gas disk, i.e., there is an appreciable contribution of secondary gas from dust grains.

Detection of submillimeter-wave [C I]emission in gaseous debris disks of 49 Ceti and β Pictoris

AbstractWe have detected [C I] 3P1–3P0 emissions in the gaseous debris disks of 49 Ceti and β Pictoris with the 10 m telescope of the Atacama Submillimeter Telescope Experiment, which is the first detection of such emissions. The line profiles of [C I] are found to resemble those of CO(J=3–2) observed with the same telescope and the Atacama Large Millimeter/submillimeter Array. This result suggests that atomic carbon (C) coexists with CO in the debris disks, and is likely formed by the photodissociation of CO. Assuming an optically thin [C I] emission with the excitation temperature ranging from 30 to 100 K, the column density of C is evaluated to be (2.2 ± 0.2) × 1017 and (2.5 ± 0.7) × 1016 cm−2 for 49 Ceti and β Pictoris, respectively. The C/CO column density ratio is thus derived to be 54 ± 19 and 69 ± 42 for 49 Ceti and β Pictoris, respectively. These ratios are higher than those of molecular clouds and diffuse clouds by an order of magnitude. The unusually high ratios of C to CO are likely attributed to a lack of H2 molecules needed to reproduce CO molecules efficiently from C. This result implies a small number of H2 molecules in the gas disk; i.e., there is an appreciable contribution of secondary gas from dust grains.

The gaseous debris disk of the white dwarf SDSS J1228+1040. HST/COS search for far-ultraviolet signatures

Context. Gaseous and dust debris disks around white dwarfs (WDs) are formed from tidally disrupted planetary bodies. This offers an opportunity to determine the composition of exoplanetary material by measuring element abundances in the accreting WD’s atmosphere. A more direct way to do this is through spectral analysis of the disks themselves. Aims. Currently, the number of chemical elements detected through disk emission-lines is smaller than that of species detected through lines in the WD atmospheres. We assess the far-ultraviolet (FUV) spectrum of one well-studied object (SDSS J122859.93+104032.9) to search for disk signatures at wavelengths Methods. We performed FUV observations (950−1240 A) with the Hubble Space Telescope/Cosmic Origins Spectrograph and used archival optical spectra. We compared them with non-local thermodynamic equilibrium model spectra. Results. No disk emission-lines were detected in the FUV spectrum, indicating that the disk effective temperature is T eff ≈ 5000 K. The long-time variability of the Ca ii IRT was reproduced with a precessing disk model of bulk Earth-like composition, having a surface mass density of 0.3 g cm -2 and an extension from 55 to 90 WD radii. The disk has a spiral shape that precesses with a period of approximately 37 years, confirming previous results.

The gaseous debris disk of the white dwarf SDSS J1228+1040

Gaseous and dust debris disks around white dwarfs (WDs) are formed from tidally disrupted planetary bodies. This offers an opportunity to determine the composition of exoplanetary material by measuring element abundances in the accreting WD's atmosphere. A more direct way to do this is through spectral analysis of the disks themselves. Currently, the number of chemical elements detected through disk emission-lines is smaller than that of species detected through lines in the WD atmospheres. We assess the far-ultraviolet (FUV) spectrum of one well-studied object (SDSS J122859.93+104032.9) to search for disk signatures at wavelengths <1050 angstrom, where the broad absorption lines of the Lyman series effectively block the WD photospheric flux. We performed FUV observations (950-1240 angstrom) with the Hubble Space Telescope/Cosmic Origins Spectrograph and used archival optical spectra. We compared them with non-local thermodynamic equilibrium model spectra. In addition, we investigate the Ca II infrared triplet (IRT) line profiles to constrain disk geometry and composition. No disk emission-lines were detected in the FUV spectrum, indicating that the disk effective temperature is about 5000K. The long-time variability of the Ca II IRT was reproduced with a precessing disk model of bulk Earth-like composition, having a surface mass density of 0.3g/cm2 and an extension from 55 to 90 WD radii. The disk has a spiral shape that precesses with a period of approximately 37 years, confirming previous results.

Exocometary gas in the HD 181327 debris ring

An increasing number of observations have shown that gaseous debris discs are not an exception. However, until now, we only knew of cases around A stars. Here we present the first detection of 12CO (2–1) disc emission around an F star, HD 181327, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with radiative transfer calculations, we study the dust and CO mass distribution. We find the dust is distributed in a ring with a radius of 86.0 ± 0.4 au and a radial width of 23.2 ± 1.0 au. At this frequency, the ring radius is smaller than in the optical, revealing grain size segregation expected due to radiation pressure. We also report on the detection of low-level continuum emission beyond the main ring out to ∼200 au. We model the CO emission in the non-local thermodynamic equilibrium regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging between 1.2 × 10−6 M⊕ and 2.9 × 10−6 M⊕, depending on the gas kinetic temperature and collisional partners densities. The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring. We derive a CO+CO2 cometary composition that is consistent with Solar system comets. Due to the low gas densities, it is unlikely that the gas is shaping the dust distribution.

SERENDIPITOUS DISCOVERY OF A CANDIDATE DEBRIS DISK AROUND THE DA WHITE DWARF SDSS J114404.74+052951.6

We report our serendipitous discovery of a candidate debris disk around the recently identified DA white dwarf (WD) SDSS J114404.74+052951.6. The Ca II infrared (IR) triplet, while weakly detected in the source's optical spectrum, shows a double-peak like profile, and near-IR excesses are also detected from broadband imaging. The two features are commonly seen in the WD gaseous debris disks, and thus indicate the existence of such a disk around the DA WD. We further analyze the emission lines and broadband IR excesses and estimate that the debris disk may have a size of 20-50 R-WD and an inclination angle of 82 degrees. The estimated temperature and cooling age for this WD are 23,027 K and similar to 21 Myr, respectively, making it possibly one of the hottest and youngest among candidates of WD debris-disk systems. These properties are also in line with the likely conditions for having a sufficiently bright debris disk. Further observations of the source are warranted in order to determine the debris disk's properties and search for more absorption features of heavy elements.

The composition of a disrupted extrasolar planetesimal at SDSS J0845+2257 (Ton 345)

We present a detailed study of the metal-polluted DB white dwarf SDSS J0845+2257 (Ton 345). Using high-resolution Hubble Space Telescope/Cosmic Origins Spectrograph and Very Large Telescope spectroscopy, we have detected hydrogen and 11 metals in the atmosphere of the white dwarf. The origin of these metals is almost certainly the circumstellar disc of dusty and gaseous debris from a tidally disrupted planetesimal, accreting at a rate of 1.6 × 1010 g s−1. Studying the chemical abundances of the accreted material demonstrates that the planetesimal had a composition similar to the Earth, dominated by rocky silicates and metallic iron, with a low water content. The mass of metals within the convection zone of the white dwarf corresponds to an asteroid of at least ∼130–170 km in diameter, although the presence of ongoing accretion from the debris disc implies that the planetesimal was probably larger than this. While a previous abundance study of the accreted material has shown an anomalously high mass fraction of carbon (15 per cent) compared to the bulk Earth, our independent analysis results in a carbon abundance of just 2.5 per cent. Enhanced abundances of core material (Fe, Ni) suggest that the accreted object may have lost a portion of its mantle, possibly due to stellar wind stripping in the asymptotic giant branch. Time series spectroscopy reveals variable emission from the orbiting gaseous disc, demonstrating that the evolved planetary system at SDSS J0845+2257 is dynamically active.

Scars of intense accretion episodes at metal-rich white dwarfs

A re-evaluation of time-averaged accretion rates at DBZ-type white dwarfs points to historical, time-averaged rates significantly higher than the currently observed episodes at their DAZ counterparts. The difference between the ongoing, instantaneous accretion rates witnessed at DAZ white dwarfs, which often exceed 1e8 g/s, and those inferred over the past 1e5-1e6 yr for the DBZ stars can be a few orders of magnitude, and therefore must result from high-rate episodes of tens to hundreds of years so they remain undetected to date. This paper explores the likelihood that such brief, intense accretion episodes of gas-phase material can account for existing data. For reasonable assumptions about the circumstellar gas, accretion rates approaching or exceeding 1e15 g/s are possible, similar to rates observed in quiescent cataclysmic variables, and potentially detectable with future x-ray missions or wide-field monitoring facilities. Gaseous debris that is prone to such rapid accretion may be abundant immediately following a tidal disruption event via collisions and sublimation, or if additional bodies impinge upon an extant disk. Particulate disk matter accretes at or near the Poynting-Robertson drag rate for long periods between gas-producing events, consistent with rates inferred for dusty DAZ white dwarfs. In this picture, warm DAZ stars without infrared excesses have rates consistent with accretion from particulate disks that remain undetected. This overall picture has implications for quasi-steady state models of accretion and the derived chemical composition of asteroidal debris in DBZ white dwarfs.

ASPITZER SPACE TELESCOPESTUDY OF THE DEBRIS DISKS AROUND FOUR SDSS WHITE DWARFS

We present Spitzer Space Telescope data of four isolated white dwarfs that were previously known to harbor circumstellar gaseous disks. IRAC photometry shows a significant infrared excess in all of the systems, SDSS0738+1835, SDSS0845+2257, SDSS1043+0855 and SDSS1617+1620, indicative of a dusty extension to those disks. The 4.5-micron excesses seen in SDSS0738, SDSS0845, and SDSS1617 are 7.5, 5.7 and 4.5 times the white dwarf contribution, respectively. In contrast, in SDSS1043, the measured flux density at 4.5 microns is only 1.7 times the white dwarf contribution. We compare the measured IR excesses in the systems to models of geometrically thin, optically thick disks, and find that we are able to match the measured SEDs to within 3 sigma of the uncertainties, although disks with unfeasibly hot inner dust temperatures generally provide a better fit than those below the dust sublimation temperature. Possible explanations for the dearth of dust around SDSS1043+0855 are briefly discussed. Including our previous study of SDSS1228+1040, all five white dwarfs with gaseous debris disks have significant amounts of dust around them. It is evident that gas and dust can coexist around these relatively warm, relatively young white dwarfs.

INTERRUPTION OF TIDAL-DISRUPTION FLARES BY SUPERMASSIVE BLACK HOLE BINARIES

Supermassive black hole binaries (SMBHBs) are products of galaxy mergers, and are important in testing Lambda cold dark matter cosmology and locating gravitational-wave-radiation sources. A unique electromagnetic signature of SMBHBs in galactic nuclei is essential in identifying the binaries in observations from the IR band through optical to X-ray. Recently, the flares in optical, UV, and X-ray caused by supermassive black holes (SMBHs) tidally disrupting nearby stars have been successfully used to observationally probe single SMBHs in normal galaxies. In this Letter, we investigate the accretion of the gaseous debris of a tidally disrupted star by a SMBHB. Using both stability analysis of three-body systems and numerical scattering experiments, we show that the accretion of stellar debris gas, which initially decays with time $\propto t^{-5/3}$, would stop at a time $T_{\rm tr} \simeq \eta T_{\rm b}$. Here, $\eta \sim0.25$ and $T_{\rm b}$ is the orbital period of the SMBHB. After a period of interruption, the accretion recurs discretely at time $T_{\rm r} \simeq \xi T_b$, where $\xi \sim 1$. Both $\eta$ and $\xi$ sensitively depend on the orbital parameters of the tidally disrupted star at the tidal radius and the orbit eccentricity of SMBHB. The interrupted accretion of the stellar debris gas gives rise to an interrupted tidal flare, which could be used to identify SMBHBs in non-active galaxies in the upcoming transient surveys.

A DUSTY COMPONENT TO THE GASEOUS DEBRIS DISK AROUND THE WHITE DWARF SDSS J1228+1040

We present ISAAC spectroscopy and ISAAC, UKIDSS and Spitzer Space Telescope broad-band photometry of SDSS J1228+1040 -- a white dwarf for which evidence of a gaseous metal-rich circumstellar disk has previously been found from optical emission lines. The data show a clear excess in the near- and mid-infrared, providing compelling evidence for the presence of dust in addition to the previously identified gaseous debris disk around the star. The infrared excess can be modelled in terms of an optically thick but geometrically thin disk. We find that the inner disk temperatures must be relatively high (~1700 K) in order to fit the SED in the near-infrared. These data provide the first evidence for the co-existence of both gas and dust in a disk around a white dwarf, and show that their presence is possible even around moderately hot (~22,000 K) stars.

SDSS J084539.17+225728.0: the first DBZ white dwarf with a metal-rich gaseous debris disc

Abstract We report the discovery of a third white dwarf hosting a gaseous debris disc, SDSS J084539.17+225728.0. The typical double-peaked Ca ii 8498,8542,8662 Å emission lines can be modelled in terms of a Keplerian gas disc with a radial extent from ∼0.5 to ∼1.0 R⊙. The effective temperature of SDSS 0845+2257, Teff≃ 18 600 ± 500 K, is comparable to the two other white dwarfs with gaseous discs, SDSS 1043+0855 and SDSS 1228+1040, and hence substantially hotter than the bulk of white dwarfs where dusty debris discs were identified through the presence of infrared excess flux. This may suggest that the conditions to produce emission lines from debris discs in the optical wavelength range are only met for a relatively narrow range in Teff. The observed asymmetry in the line profiles indicates a substantial eccentricity in the disc. Two spectra obtained four years apart reveal a significant change in the shapes and equivalent widths of the line profiles, implying that the circumstellar disc evolves on relatively short time-scales. In contrast to SDSS 1043+0855 and SDSS 1228+1040, SDSS 0845+2257 has a helium-dominated atmosphere. We detect photospheric absorption lines of He, Ca, Mg and Si in the Sloan Digital Sky Survey spectrum, and hence classify SDSS 0845+2257 as DBZ white dwarf. The abundances for the three metals determined from model atmosphere fits are Ca/He ≃ 1.3 × 10−7, Mg/He ≃ 6.0 × 10−6 and Si/He ≃ 8.0 × 10−6. From the non-detection of Hα, we derive H/He &amp;lt; 3 × 10−5, which implies that the hydrogen-to-metal abundance ratio of the circumstellar material is ≳1000 times lower than in the Sun. This lends strong support to the hypothesis that the gaseous and dusty debris discs found around roughly a dozen white dwarfs originate from the disruption of rocky planetary material.

SDSS J104341.53+085558.2: a second white dwarf with a gaseous debris disc

Abstract Intermediate-resolution spectroscopy of the white dwarf SDSS J104341.53+085558.2 (SDSS J1043+0855) contains double-peaked emission lines of Ca iiλλ8498, 8542, 8662 and identifies this object to be the second single white dwarf to be surrounded by a gaseous disc of metal-rich material, similar to the recently discovered SDSS J1228+1040. A photospheric magnesium abundance of 0.3 times the solar value, determined from the observed Mg iiλ4481 absorption line, implies that the white dwarf is accreting from the circumstellar material. The absence of Balmer emission lines and of photospheric He iλ4471 absorption indicates that the accreted material is depleted in volatile elements and, by analogy with SDSS J1228+1040, may be the result of the tidal disruption of an asteroid. Additional spectroscopy of the DAZ white dwarfs WD 1337+705 and GD 362 does not reveal Ca ii emission lines. GD 362 is one of the few cool DAZ white dwarfs that display strong infrared flux excess, thought to be originating in a circumstellar dust disc, and its temperature is probably too low to sublimate sufficient amounts of disc material to generate detectable Ca ii emission. WD 1337+705 is, like SDSS J1228+1040 and SDSS J1043+0855, moderately hot, but has the lowest Mg abundance of those three stars, suggesting a possible correlation between the photospheric Mg abundance and the equivalent width of the Ca ii emission triplet. Our inspection of 7360 white dwarfs from the Sloan Digital Sky Survey Data Release 4 fails to unveil additional strong ‘metal gas disc’ candidates, and implies that these objects are rather rare.

Roche Lobe Overflow from Dwarf Stellar Systems

We use both analytical analyses and numerical simulations to examine the evolution of residual gas within tidally-limited dwarf galaxies and globular clusters. If the gas sound speed exceeds about 10% of the central velocity dispersion, as is the case for ionized gas within small stellar systems, the gas shall have significant density at the tidal radius, and the gas may be lost on timescales as short as a few times the sound crossing time of the system. In colder systems, the density at the tidal radius is much lower, greatly reducing the mass loss rate, and the system may retain its gas for a Hubble time. The tidally removed gas shall follow an orbit close to that of the original host system, forming an extended stream of ionized, gaseous debris. Tidal mass loss severely limits the ability of dwarf systems to continuously form stars. The ordinary gas content in many dwarf galaxies is fully ionized during high red-shift epochs, possibly preventing star formation in some systems, leading to the formation of starless, dark-matter concentrations. In either the field or in the center of galaxy clusters, ionized gas may be retained by dwarf galaxies, even though its sound speed may be comparable to or even exceed the velocity dispersion. These processes may help to explain some observed differences among dwarf galaxy types, as well as observations of the haloes of massive galaxies.

The role of electromagnetic radiation in hypervelocity impact mechanics

This paper is based upon an acceptance presentation I made for the 1998 Distinguished Scientist award presented by the Hypervelocity Impact Society. Nearly thirty years ago, I recorded a cine sequence of a gaseous debris plume expanding behind a thin cadmium plate impacted by a cadmium sphere. The plume stagnated on a second plate where it glowed so brightly that it must have been heated to over 50,000 oK. The fact went unnoticed for nearly twenty years until it was predicted theoretically and then finally observed. The theoretical analysis showed that virtually all gaseous impact debris that is intercepted gives up its energy to electromagnetic radiation. Potential exists to use the resulting radiation for a variety of research opportunities. More recently, tungsten plasma accelerated electrically with a 10 MV pulse generator at Sandia National Laboratories has been imploded onto the axis of an evacuated cylindrical cavity where it stagnates against itself to produce enormous pulses of incoherent electromagnetic radiation. The intensity of the radiation is so great that it vaporizes and launches surface material off samples exposed to it. Extremely intense shockwaves are produced that are being used for extending material equation-of-state determinations well above pressure levels reachable by laboratory impacts.

Optical Structure in the Abell 1795 Cluster Central Galaxy: Evidence for Stripping and Deflection of Radio Jets

We have obtained a deep U-band image of the central ~100 kpc of the Abell 1795 cluster central galaxy. This image shows several faint (μU ~ 23-24 mag arcsec-2), thin structures, of unknown origin, that extend between 20 and 70 kpc into the galaxy's halo. Some are located along filaments of nebular line emission. Two structures, detected in the ground-based U-band image and in V and R images obtained with the WFPC2 on the Hubble Space Telescope (HST), extend toward faint, disturbed galaxies. The structures may be stripped stellar and gaseous debris from several sub-L* galaxies that are interacting with the central dominant galaxy. The lobes of blue optical continuum found earlier along the galaxy's radio lobes are resolved into bright knots on the HST images. The knots are probably composed of young stars. A dust lane lies along the edges of the radio jets and along the edge of the northern radio lobe. The dust and associated cold, dense gas may have deflected the radio jets and may have become entrained along the radio jets and lobes. The images strengthen recent arguments for radio-triggered star formation in A1795, and they raise the possibility that some of the accreted gas may have been stripped from neighboring galaxies.