The Decay of Debris Disks around Solar-Type Stars

We present a Spitzer MIPS study of the decay of debris disk excesses at 24 and 70 $\mu$m for 255 stars of types F4 - K2. We have used multiple tests, including consistency between chromospheric and X-ray activity and placement on the HR diagram, to assign accurate stellar ages. Within this spectral type range, at 24 $\mu$m, $13.6 \pm 2.8 \%$ of the stars younger than 5 Gyr have excesses at the 3$\sigma$ level or more, while none of the older stars do, confirming previous work. At 70 $\mu$m, $22.5 \pm 3.6\%$ of the younger stars have excesses at $\ge$ 3 $\sigma$ significance, while only $4.7^{+3.7}_{-2.2}$% of the older stars do. To characterize the far infrared behavior of debris disks more robustly, we double the sample by including stars from the DEBRIS and DUNES surveys. For the F4 - K4 stars in this combined sample, there is only a weak (statistically not significant) trend in the incidence of far infrared excess with spectral type (detected fractions of 21.9$^{+4.8}_{-4.3}\%$, late F; 16.5$^{+3.9}_{-3.3}\%$, G; and 16.9$^{+6.3}_{-5.0}\%$, early K). Taking this spectral type range together, there is a significant decline between 3 and 4.5 Gyr in the incidence of excesses with fractional luminosities just under $10^{-5}$. There is an indication that the timescale for decay of infrared excesses varies roughly inversely with the fractional luminosity. This behavior is consistent with theoretical expectations for passive evolution. However, more excesses are detected around the oldest stars than is expected from passive evolution, suggesting that there is late-phase dynamical activity around these stars.Comment: 46 pages. 7 figures. Accepted to Ap

Common Warm Dust Temperatures Around Main-sequence Stars

We compare the properties of warm dust emission from a sample of main-sequence A-type stars (B8-A7) to those of dust around solar-type stars (F5-K0) with similar Spitzer Space Telescope Infrared Spectrograph/MIPS data and similar ages. Both samples include stars with sources with infrared spectral energy distributions that show evidence of multiple components. Over the range of stellar types considered, we obtain nearly the same characteristic dust temperatures (~190 K and ~60 K for the inner and outer dust components, respectively)—slightly above the ice evaporation temperature for the inner belts. The warm inner dust temperature is readily explained if populations of small grains are being released by sublimation of ice from icy planetesimals. Evaporation of low-eccentricity icy bodies at ~150 K can deposit particles into an inner/warm belt, where the small grains are heated to T_(dust)~ 190 K. Alternatively, enhanced collisional processing of an asteroid belt-like system of parent planetesimals just interior to the snow line may account for the observed uniformity in dust temperature. The similarity in temperature of the warmer dust across our B8-K0 stellar sample strongly suggests that dust-producing planetesimals are not found at similar radial locations around all stars, but that dust production is favored at a characteristic temperature horizon

Spitzer Observations of Low Luminosity Isolated and Low Surface Brightness Galaxies

We examine the infrared properties of five low surface brightness galaxies (LSBGs) and compare them with related but higher surface brightness galaxies, using Spitzer Space Telescope images and spectra. All the LSBGs are detected in the 3.6 and 4.5um bands, representing the stellar population. All but one are detected at 5.8 and 8.0um, revealing emission from hot dust and aromatic molecules, though many are faint or point-like at these wavelengths. Detections of LSBGs at the far-infrared wavelengths, 24, 70, and 160um, are varied in morphology and brightness, with only two detections at 160um, resulting in highly varied spectral energy distributions. Consistent with previous expectations for these galaxies, we find that detectable dust components exist for only some LSBGs, with the strength of dust emission dependent on the existence of bright star forming regions. However, the far-infrared emission may be relatively weak compared with normal star-forming galaxies.Comment: 20 pages, 8 figures, accepted to Ap

Mid-Infrared Emission from E+A Galaxies in the Coma Cluster

We have used ISO to observe at 12$\mu$m seven E+A galaxies plus an additional emission line galaxy, all in the Coma cluster. E+A galaxies lacking narrow emission lines have 2.2$\mu$m to 12$\mu$m flux density ratios or limits similar to old stellar populations (typical of early-type galaxies). Only galaxies with emission lines have enhanced 12$\mu$m flux density. Excess 12$\mu$m emission is therefore correlated with the presence of on-going star formation or an active galactic nucleus (AGN). By comparing the current star formation rates with previous rates estimated from the Balmer absorption features, we divide the galaxies into two groups: those for which star formation has declined significantly following a dramatic peak $\sim$ 1 Gyr ago; and those with a significant level of ongoing star formation or/and an AGN. There is no strong difference in the spatial distribution on the sky between these two groups. However, the first group has systemic velocities above the mean cluster value and the second group below that value. This suggests that the two groups differ kinematically. Based on surveys of the Coma cluster in the radio, the IRAS sources, and galaxies detected in H$\alpha$ emission, we sum the far infrared luminosity function of galaxies in the cluster. We find that star formation in late type galaxies is probably the dominant component of the Coma cluster far infrared luminosity. The presence of significant emission from intracluster dust is not yet firmly established. The member galaxies also account for most of the far infrared output from nearby rich clusters in general.Comment: AAS Latex, accepted for publication in Ap

Infrared Emission by Dust Around lambda Bootis Stars: Debris Disks or Thermally Emitting Nebulae?

We present a model that describes stellar infrared excesses due to heating of the interstellar (IS) dust by a hot star passing through a diffuse IS cloud. This model is applied to six lambda Bootis stars with infrared excesses. Plausible values for the IS medium (ISM) density and relative velocity between the cloud and the star yield fits to the excess emission. This result is consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A- to F-type stars with large underabundances of Fe-peak elements) owe their characteristics to interactions with the ISM. This proposal invokes radiation pressure from the star to repel the IS dust and excavate a paraboloidal dust cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar photosphere. However, the measurements of the infrared excesses can also be fit by planetary debris disk models. A more detailed consideration of the conditions to produce lambda Bootis characteristics indicates that the majority of infrared-excess stars within the Local Bubble probably have debris disks. Nevertheless, more distant stars may often have excesses due to heating of interstellar material such as in our model.Comment: 10 pages, 5 figures, 4 tables, accepted by ApJ, emulateap

Near-Infrared and Star-forming properties of Local Luminous Infrared Galaxies

We use HST NICMOS continuum and Pa-alpha observations to study the near-infrared and star-formation properties of a representative sample of 30 local (d ~ 35-75Mpc) luminous infrared galaxies (LIRGs, infrared 8-1000um luminosities of L_IR=11-11.9[Lsun]). The data provide spatial resolutions of 25-50pc and cover the central ~3.3-7.1kpc regions of these galaxies. About half of the LIRGs show compact (~1-2kpc) Pa-alpha emission with a high surface brightness in the form of nuclear emission, rings, and mini-spirals. The rest of the sample show Pa-alpha emission along the disk and the spiral arms extending over scales of 3-7kpc and larger. About half of the sample contains HII regions with H-alpha luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24um and hydrogen recombination (extinction-corrected Pa-alpha) luminosity for these LIRGs, and the HII regions in the central part of M51. This relation holds over more than four decades in luminosity suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of Kennicutt (1998), we derive an empirical calibration of the SFR in terms of the monochromatic 24um luminosity that can be used for luminous, dusty galaxies.Comment: Accepted for publication in ApJ. Contact first author for high qualitity version of figure

Quasars and Ultraluminous Infrared Galaxies: At the Limit?

We have detected the host galaxies of 16 nearby, radio-quiet quasars using images obtained with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). We confirm that these luminous quasars tend to live in luminous, early-type host galaxies, and we use the host-galaxy magnitudes to refine the luminosity/host-mass limit inferred from ground-based studies. If quasars obey the relation $M_{black hole}/M_{spheroid}\sim0.006$ found for massive dark objects in nonactive galaxies, then our analysis implies that they radiate at up to $\sim20$ of the Eddington rate. An analogous analysis for ultraluminous infrared galaxies shows them to accrete at up to similar Eddington fractions, consistent with the hypothesis that some of them are powered by embedded quasars.Comment: 9 pages, includes 2 eps figs, accepted to ApJLet

Extended Emission by Dust in the Dwarf Galaxy UGC 10445

We present Spitzer Space Telescope images of the isolated dwarf galaxy UGC 10445. The galaxy is detected at all photometric bands (3.6-160um) as well as in the Multiband Imaging Photometer for Spitzer (MIPS) spectral energy distribution mode (55-95um). We derive a star formation rate of 0.25 M_sun/yr based on H-alpha and infrared flux densities. There is over 10^6 solar masses of cold dust (T~18K) in the galaxy, represented by 160um emission, that extends to a larger radius than the ultraviolet (UV), optical and near-infrared light. Such extended emission has been seen previously only in dwarf galaxies in cluster environments. We suggest the source of heating for this dust is UV light originating in star forming complexes. To produce the large quantity of dust requires a higher rate of star formation in the past than is observed currently.Comment: 11 pages, 5 pages, accepted to ApJ, color high res figures available upon reques