arXiv subjects

M. W. Werner

Publications and source records attributed to M. W. Werner.

51 records · Page 3Linked to original sources

arXiv2001

What next for the Likely Pre-Supernova, HD 179821?

We have used the Owens Valley Radio Observatory Millimeter Array to obtain a map of the J = (1-0) CO emission from the circumstellar shell around HD 179821, a highly evolved G-type star which will probably explode as a supernova in the next 100,000 yr. Very approximately, the gas presents as a circular ring with azimuthal variations in the CO brightness by about a factor of 2. Until about 1600 years ago, the star was a red hypergiant losing about 0.0003 M(Sun)/yr at an average outflow speed of 32 km/s. We propose that when HD 179821 explodes as a supernova, it may resemble Kepler's supernova remnant and thus some of the anisotropies in supernova remnants may be intrinsic. If the factors which cause the anisotropic mass loss in HD 179821 persist to the moment when the star explodes as a supernova, the newly-born pulsar may receive a momentum "kick" leading to a space motion near 700 km/s. Independent of the angular asymmetries, the radially detached shell around HD 179821 may be representative of environments which produce dust echoes from gamma-ray bursts.

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arXiv2000

The Dying Wind Around HD 56126, A Post-AGB Carbon Star

We have used the Keck I telescope to resolve at mid-infrared wavelengths the dust emission from HD 56126 (IRAS 07134+1005), a post-Asymptotic Giant Branch carbon star with a detached dust shell. The gross morphology of the image can be explained by a strong wind which started to die about 1500 years ago. If the star had an effective temperature near 2600 K when it was losing a large amount of mass, then during the past 1500 years, the average value of dT/dt has been +2.2 K/yr. With such a time variation of the effective temperture of the star, the 11.7 micron image can be approximately reproduced if the mass loss rate varied as T^(-8.26), as proposed in recent models for dust-driven winds. Since the mass loss rate appears to be very sensitive to the effective temperature of the star, we speculate that the observed deviations from spherical symmetry of the dust shell can be explained by plausible variations in the surface temperature of the mass-losing star caused by rotation and/or magnetic fields.

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arXiv2000

High Spatial Resolution Imaging of NGC 1068 in the Mid Infrared

Mid-infrared observations of the central source of NGC 1068 have been obtained with a spatial resolution in the deconvolved image of 0.1" (~ 7pc). The central source is extended by ~1" in the north-south direction but appears unresolved in the east-west direction over most of its length. About two-thirds of its flux can be ascribed to a core structure which is itself elongated north-south and does not show a distinct unresolved compact source. The source is strongly asymmetric, extending significantly further to the north than to the south. The morphology of the mid-infrared emission appears similar to that of the radio jet, and has features which correlate with the images in [O III]. Its 12.5-24.5 micron color temperature ranges from 215 to 260 K and does not decrease smoothly with distance from the core. Silicate absorption is strongest in the core and to the south and is small in the north. The core, apparently containing two-thirds of the bolometric luminosity of the inner 4" diameter area, may be explained by a thick, dusty torus near the central AGN viewed at an angle of ~65 deg to its plane. There are, however, detailed difficulties with existing models, especially the narrow east-west width of the thin extended mid-infrared "tongue" to the north of the core. We interpret the tongue as re-processed visual and ultraviolet radiation that is strongly beamed and that originates in the AGN.

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arXiv2000

The Mid-Infrared Spectra of Normal Galaxies

The mid-infrared spectra (2.5 to 5 and 5.7 to 11.6 mu) obtained by ISO-PHOT reveal the interstellar medium emission from galaxies powered by star formation to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6 and 11.3 mu. Additional emission appears in-between the features, and an underlying continuum is clearly evident at 3-5 mu. This continuum would contribute about a third of the luminosity in the 3 to 13 mu range. The features together carry 5 to 30% of the 40-to-120 mu `FIR' luminosity. The relative fluxes in individual features depend very weakly on galaxy parameters such as the far-infrared colors, direct evidence that the emitting particles are not in thermal equilibrium. The dip at 10 mu is unlikely to result from silicate absorption, since its shape is invariant among galaxies. The continuum component has a f_nu \~ nu^{0.65} shape between 3 and 5 mu and carries 1 to 4% of the FIR luminosity; its extrapolation to longer wavelengths falls well below the spectrum in the 6 to 12 mu range. This continuum component is almost certainly of non-stellar origin, and is probably due to fluctuating grains without aromatic features. The spectra reported here typify the integrated emission from the interstellar medium of the majority of star-forming galaxies, and could thus be used to obtain redshifts of highly extincted galaxies up to z=3 with SIRTF.

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arXiv1999

High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies

Observations of ultraluminous infrared galaxies (ULIRGs) with an achieved resolution approaching the diffraction limit in the mid-infrared from 8 - 25 $μ$m using the Keck Telescopes are reported. We find extremely compact structures, with spatial scales of $< 0.3''$ (diameter) in six of the seven ULIRGs observed. These compact sources emit between 30% and 100% of the mid-infrared energy from these galaxies. We have utilized the compact mid-infrared structures as a diagnostic of whether an AGN or a compact (100 -- 300 pc) starburst is the primary power source in these ULIRGs. In Markarian 231, the upper limit on the diameter of the 12.5 $μ$m source, 0.13$''$, shows that the size of the infrared source must increase with increasing wavelength, consistent with AGN models. In IRAS 05189-2524 and IRAS 08572+3915 there is strong evidence that the source size increases with increasing wavelength. This suggests heating by a central source rather than an extended luminosity source, consistent with the optical classification as an AGN. The compact mid-infrared sources seen in the other galaxies cannot be used to distinguish the ultimate luminosity source. If these ULIRGs are powered by compact starbursts, the star formation rates seen in the central few hundred parsecs far exceed the global rates seen in nearby starburst galaxies, and approach the surface brightness of individual clusters in nearby starburst galaxies.

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arXiv1999

ISO Mid-Infrared Spectra of Reflection Nebulae

We present 5-15 micron imaging spectroscopy of the reflection nebulae vdB 17 (NGC 1333), vdB 59 (NGC 2068), vdB 101, vdB 111, vdB 133 and vdB 135, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory (ISO). These nebulae are illuminated by stars with Teff = 3,600-19,000 K, implying ultraviolet to total stellar flux ratios of F(< 400 nm)/F(tot) = 0.01-0.87. We detect the infrared emission features (IEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 microns, broad emission features at 6-9 microns and 11-13 microns, and 5-15 micron continuum emission, from the interstellar medium in vdB 17, vdB 59, and vdB 133 (F(< 400 nm)/F(tot) = 0.22-0.87), and place upper limits on the emission from the interstellar medium in vdB 101, vdB 111, and vdB 135 (F(< 400 nm)/F(tot) = 0.01-0.20). We observe no spectroscopic differences with varying Teff. We analyze the spectra of vdB 17 and vdB 59 as a function of distance from the star to see how the spectra depend on Go within each source. The only quantitative difference we find is a broadening of the 7.7 micron IEF at Go = 20-60 within vdB 17. We observe only a 40% change in the 6-10 micron to 10-14 micron flux ratio over Go = 20 to 60,000.

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arXiv1998

High Spatial Resolution Imaging of Arp 220 from 3 - 25 microns

Images of Arp 220 from 3.45 to 24.5 $μ$m with 0.5'' resolution are presented which clearly separate the nucleus into at least two components. The western component is about three times more luminous than the eastern component, but the silicate absorption in the fainter, eastern component is roughly 50% greater than the absorption in the western component. Each component is marginally resolved. The two components seen at 24.5 $μ$m are identified with the two radio components. The western source most likely coincides with the high extinction disk previously suggested to exist in Arp 220, while the eastern nucleus is identified with a faint highly reddened source seen in HST 2.2 $μ$m NICMOS images. The two nuclei together account for essentially all of the measured 24.5 $μ$m flux density. Two models are presented, both of which fit the observations. In one, the majority of the total luminosity is produced in an extended star formation region and in the other most of the luminosity is produced in the compact but extincted regions associated with the two nuclei seen at 24.5 $μ$m. In both pictures, substantial luminosity at 100 $μ$m emerges from a component having a diameter of 2--3'' ($\sim$1kpc).

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arXiv1998

High Resolution Mid-Infrared Imaging of the Nucleus of NGC 1068

We have obtained mid-infrared images of the nucleus of NGC 1068 from the Hale 5 m telescope at Mt. Palomar with diffraction-limited resolution and high sensitivity at 8.8, 10.3, and 12.5 microns. Deconvolved images show that the infrared emission extends north to south in the inner 2", consisting of a central peak, a component extending 1" north of the central source, a component extending 1" south of the central source, and several smaller structures located 1" to the northeast. The central peak is extended 0.4" N-S and unresolved (<= 0.2") E-W. We find that 50 +/- 5% of the flux emerges from the central 0.4" and that a single unresolved point source can account for only 27 +/- 5% of the total flux. However, if the central peak arises from optically thick emission, we estimate that the emitting region has a projected area >= 2 pc^2, and may thus contain a compact source such as a parsec-scale torus. We observe a correspondence between the northern extension and the northeastern sources appearing on the mid-infrared images and the [OIII] clouds A-C & E. We interpret the faint optical counterpart to the mid-infrared southern extension as being due to partial obscuration by the intervening disk of the host galaxy. The N-S extension of the mid-infrared emission coincides with one wall of the conical narrow line region and aligns with the N-S orientation of the radio jet close to the nucleus. We interpret the infrared emission as arising from optically thick dust lining the walls of the low density cavity formed by the radio jet and heated by radiation from the central source.

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arXiv1998

Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation

We report the discovery of a circumstellar disk around the young A0 star, HR 4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By fitting a model of the emission from a flat dusty disk to an image at lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission does not decrease with radius as expected for circumstellar disks but increases outward from the star, peaking near both ends of the elongated structure. We simulate this appearance by varying the inner radius in our model and find an inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to the radial distance of our own Kuiper belt and may suggest a source of dust in the collision of cometesimals. By contrast with the appearance at 20.8 microns, excess emission at lambda = 12.5 microns is faint and concentrated at the stellar position. Similar emission is also detected at 20.8 microns in residual subtraction of the best-fit model from the image. The intensity and ratio of flux densities at the two wavelengths could be accounted for by a tenuous dust component that is confined within a few AU of the star with mean temperature of a few hundred degrees K, similar to that of zodiacal dust in our own solar system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that its disk is in a transitional planet-forming stage, between that of massive gaseous proto-stellar disks and more tenuous debris disks such as the one detected around Vega.

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arXiv1997

Do the Infrared Emission Features Need Ultraviolet Excitation

We present the results of imaging spectroscopy of the reflection nebula vdB 133, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory (ISO). Our observations reveal the infrared emission features (IEFs), at 6.2, 7.7, 8.6, 11.3, and 12.7 micron, and associated 5 -- 15 micron continuum emission. The stellar system illuminating vdB 133 has the lowest ratio of ultraviolet (shortward of 0.4 micron) to total flux of any stars demonstrated to date to excite the IEFs and associated continuum emission from adjacent interstellar dust, as opposed to circumstellar dust. The low fraction of UV flux from this system poses a problem for existing models for the emission mechanism and emitting material, which all require substantial UV radiation for the excitation of the IEFs and associated continuum.

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arXiv1997

ISO measurements of [CII] line variations in galaxies

We report measurements of the [CII] fine structure line at 157.714 micron in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared luminosity, [CII]/FIR, measures the ratio of the cooling of gas to that of dust; and thus the efficiency of the grain photoelectric heating process. This ratio varies by a factor of 40 in the current sample. About two-thirds of the galaxies have [CII]/FIR = 2-7 x 10^{-3}. The other one-third show trends of decreasing [CII]/FIR with increasing dust temperature, indicated by the ratio of infrared emission at 60 and 100 microns; and with increasing star-formation activity, measured by the ratio of far-infrared and blue band luminosity. We find three FIR bright galaxies with undetected [CII] line at 3-sigma upper limits of [CII]/FIR < 0.5-2 x 10^{-4}. The trend in the [CII]/FIR ratio with the temperature of dust and with star-formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed PDR lines have among the highest ratios of FIR to Blue luminosity and the ratio of 60 and 100 micron emission. Their lack of [CII] lines may be due to a continuing trend of decreasing [CII]/FIR with increasing star-formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case the upper limits on [CII]/FIR imply a ratio of UV flux to gas density G_0/n > 10 cm$^3. The low [CII]/FIR could also be due to either weak [CII] because of self-absorption or strong FIR continuum from regions weak in [CII], such as dense HII regions or plasma ionized by hard radiation of AGNs.

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arXiv1996

ISOCAM Observations of NGC 6946: Mid-IR Structure

The nearby spiral galaxy NGC 6946 was observed with ISO-CAM in the mid-infrared, achieving 7 arcsec resolution and sub-MJy/steradian sensitivity. Images taken with CAM filters LW2 (7 microns) and LW3 (15 microns) are analysed to determine the morphology of this galaxy and understand better the emission mechanisms. The mid-infrared emission follows an exponential disk with a scale length 75 arcsec. This is 60% of the scale length in the optical R-band and radio continuum. The nuclear starburst region is too bright for reliable measurement in these images. Its surface brightness exceeds the inner disk by at least a factor of 12. The arms and interarm regions are clearly detected, with each of these components contributing about equally to the disk emission. The arm-interarm contrast is 2-4 in the mid-IR, close to that measured in the visible R band light and lower than the contrast in $Hα$, suggesting that non-ionizing radiation contributes significantly to dust heating.

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arXiv1996

The Mid-Infrared Color of NGC 6946

We analyze the new mid-infrared maps of NGC 6946 for variations in the color ratio of the 7-to-15 micron emission. Our preliminary findings are that this mid-infrared color is remarkably constant between arms and inter-arm regions, and as a function of radius in the disk, excluding the nuclear region. As surface brightness ranges by more than an order of magnitude and the radius runs from about 0.5 to 3kpc, the color ratio remains constant to about +/-20%. Our interpretation is that (1) hard UV radiation from OB stars does not dominate the heating of the grains radiating in the mid-infrared; and (2) that surface brightness variations are driven primarily by surface-filling fraction in the disk, and by radiation intensity increases in starburst environments, such as the nucleus of NGC 6946.

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arXiv1996

An ISOPHOT of the disk of galaxy NGC 6946: 60 micron infrared and radio continuum correlation

We use the ISOPHOT 60 micron image of Tuffs et al. (1996) to study the 60 micron light distribution and its correlation with radio continuum within the disk of spiral galaxy NGC6946, and to resolve the long-standing IRAS controversy as to whether the infrared-to-radio continuum ratio, Q, decreases radially out in galaxy disks. Our main results are: (1) The disk at 60 microns also appears to follow an exponential fall-off. Its e-folding scale-length lies between 1'.3 and an upper limit of 1'.9, obtained from the mean radial profile between 1'.5 and 4' in radius. This scale length is smaller than that derived from the radio continuum data at either 20cm or 6cm, with or without the thermal component removed, at a significance level of >~ 2 sigma. From 1'.5 to 4' in radius, the 60 micron -to-radio continuum surface brightness ratio, Q, decreases by a factor of 1.5 to 1.7 on average. These quantitative results agree with that of Marsh & Helou (1995) using IRAS HiRes data, but not with that of Fitt et al. (1992) based on IRAS CPC data. Our results also fit into the picture of the radio disk as a ``smeared'' version of the infrared disk, but are not consistent with the picture in which the variation of Q is a result of the thermal and non-thermal radio components having two distinct, constant values of the infrared-to-radio ratio.

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arXiv1995

A Survey of Near Infrared Emission in Visual Reflection Nebulae

We present a survey for extended 2.2 $μ$m emission in 20 new visual reflection nebulae, illuminated by stars with temperatures of 3,600 --- 33,000 K. We detect extended 2.2 $μ$m emission in 13 new nebulae, illuminated by stars with temperatures of 6,800 -- 33,000 K. For most of these 13 nebulae we have measured $J-K$, $H-K$, and $K-L'$, as well as obtaining surface brightness measurements at the wavelength of the 3.3 $μ$m emission feature. All of the reflection nebulae with extended near infrared emission in excess over scattered starlight have very similar near infrared colors and show the 3.3 $μ$m feature in emission with similar feature-to-continuum ratios. The 3.3 $μ$m feature-to-continuum ratio ranges from $\sim$3 to $\sim$9, both within individual nebulae and from nebula to nebula, which suggests that the 3.3 $μ$m feature and its underlying continuum arise from different materials, or from different ranges of sizes within a size distribution of particles. No dependence on the temperature of the illuminating star is seen in the near infrared colors or 3.3 $μ$m feature-to-continuum ratio, over a factor of two in stellar temperature. This is similar to our previous IRAS results, in which we found no dependence of the ratio of 12 $μ$m to 100 $μ$m surface brightnesses in reflection nebulae illuminated by stars with temperatures of 5,000--33,000 K.

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