arXiv subjects

Charles E. Woodward

Publications and source records attributed to Charles E. Woodward.

85 records · Page 5Linked to original sources

arXiv2006

A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)

We present infrared images and spectra of comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT) as part of a larger program to observe comets inside of 5 AU from the sun with the Spitzer Space Telescope. The nucleus of comet 2P/Encke was observed at two vastly different phase angles (20 degrees and 63 degrees). Model fits to the spectral energy distributions of the nucleus suggest comet Encke's infrared beaming parameter derived from the near-Earth asteroid thermal model may have a phase angle dependence. The observed emission from comet Encke's dust coma is best-modeled using predominately amorphous carbon grains with a grain size distribution that peaks near 0.4 microns, and the silicate contribution by mass to the sub-micron dust coma is constrained to 31%. Comet 67P/Churyumov-Gerasimenko was observed with distinct coma emission in excess of a model nucleus at a heliocentric distance of 5.0 AU. The coma detection suggests that sublimation processes are still active or grains from recent activity remain near the nucleus. Comet C/2001 HT50 (LINEAR-NEAT) showed evidence for crystalline silicates in the spectrum obtained at 3.2 AU and we derive a silicate-to-carbon dust ratio of 0.6. The ratio is an order of magnitude lower than that derived for comets 9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 (Hale-Bopp).

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arXiv2006

Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula

We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and High-resolution Spitzer IRS spectra of selected positions within the nebula, and a near-infrared ground-based image made in the light of [Fe II]1.644 micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0 micron, resembles the general morphology of visible H-alpha and near-IR [Fe II] line emission, while the 3.6 and 4.5 micron images are dominated by continuum synchrotron emission. The 24 micron and 70 micron images show enhanced emission that may be due to line emission or the presence of a small amount of warm dust in the nebula on the order of less than 1% of a solar mass. The ratio of the 3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power law index ranging from approximately 0.3 to 0.8 across the nebula. Combining this information with optical and X-ray synchrotron images, we derive a broadband spectrum that reflects the superposition of the flatter spectrum jet and torus with the steeper diffuse nebula, and suggestions of the expected pileup of relativistic electrons just before the exponential cutoff in the X-ray. The pulsar, and the associated equatorial toroid and polar jet structures seen in Chandra and HST images (Hester et al. 2002) can be identified in all of the IRAC images. We present the IR photometry of the pulsar. The forbidden lines identified in the high resolution IR spectra are all double due to Doppler shifts from the front and back of the expanding nebula and give an expansion velocity of approximately 1264 km/s.

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arXiv2006

Stellar Populations and Mass-Loss in M15: A Spitzer Detection of Dust in the Intra-Cluster Medium

We present Spitzer Space Telescope IRAC and MIPS observations of the galactic globular cluster M15 (NGC 7078), one of the most metal-poor clusters with a [Fe/H] = -2.4. Our Spitzer images reveal a population of dusty red giants near the cluster center, a previously detected planetary nebula (PN) designated K648, and a possible detection of the intra-cluster medium (ICM) arising from mass loss episodes from the evolved stellar population. Our analysis suggests 9 (+/-2) x 10^-4 solar masses of dust is present in the core of M15, and this material has accumulated over a period of approximately 10^6 years, a timescale ten times shorter than the last galactic plane crossing event. We also present Spitzer IRS follow up observations of K648, including the detection of the [NeII] 12.81 micron line, and discuss abundances derived from infrared fine structure lines.

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arXiv2006

Spitzer Far-Infrared Detections of Cold Circumstellar Disks

Observations at 70 microns with the Spitzer Space Telescope have detected several stellar systems within 65 pc of the Sun. Of 18 presumably young systems detected in this study, as many as 15 have 70-micron emission in excess of that expected from their stellar photospheres. Five of the systems with excesses are members of the Tucanae Association. The 70-micron excesses range from a factor of ~2 to nearly 30 times the expected photospheric emission from these stars. In contrast to the 70-micron properties of these systems, there is evidence for an emission excess at 24 microns for only HD 3003, confirming previous results for this star. The lack of a strong 24-micron excess in most of these systems suggests that the circumstellar dust producing the IR excesses is relatively cool (T_dust < 150 K) and that there is little IR-emitting material within the inner few AU of the primary stars. Many of these systems lie close enough to Earth that the distribution of the dust producing the IR excesses might be imaged in scattered light at optical and near-IR wavelengths.

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arXiv2006

On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass

We report 4.5 micron luminosities for 27 nearby (D < 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 micron luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 micron L-Z relation is 12+log(O/H) = (5.78 +/- 0.21) + (-0.122 +/- 0.012) M_[4.5], where M_[4.5] is the absolute magnitude at 4.5 micron. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity M-Z relation is 12+log(O/H) = (5.65 +/- 0.23) + (0.298 +/- 0.030) log Mstar. and extends the SDSS M-Z relation to lower mass by about 2.5 dex. We find that the dispersion in the M-Z relation is similar over five orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L-Z and M-Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed.

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arXiv2006

Hot Dust and PAH Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies

We present Spitzer Space Telescope 4.5 and 8.0 micron imaging of 15 Local Group and nearby dwarf galaxies. Our sample spans a range of more than one dex in nebular metallicity and over three orders of magnitude in current star formation rate, allowing us to examine the dependence of the diffuse 8 micron emission, originating from hot dust and PAHs, on these parameters. We detect prominent diffuse 8 micron emission in four of the most luminous galaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109), low surface brightness emission from four others (DDO 216, Sextans A, Sextans B, WLM), and no diffuse emission from the remaining objects. We observe general correlations of the diffuse 8 micron emission with both the current star formation rate and the nebular metallicity of the galaxies in our sample. However, we also see exceptions to these correlations that suggest other processes may also have a significant effect on the generation of hot dust/PAH emission. These systems all have evidence for old and intermediate age star formation, thus the lack of diffuse 8 micron emission cannot be attributed to young galaxy ages. Also, we find that winds are unlikely to explain the paucity of diffuse 8 micron emission, since high resolution imaging of the neutral gas in these objects show no evidence of blowout. Additionally, we propose the lack of diffuse 8 micron emission in low-metallicity systems may be due to the destruction of dust grains by supernova shocks, assuming the timescale to regrow dust grains and PAH molecules is long compared to the destruction timescale. The most likely explanation for the observed weak diffuse 8 micron emission is at least partly due to a general absence of dust (including PAHs), in agreement with their low metallicities.

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arXiv2006

The first 8-13 micron spectra of globular cluster red giants: circumstellar silicate dust grains in 47 Tucanae (NGC 104)

We present 8-13 micron spectra of eight red giants in the globular cluster 47 Tucanae (NGC 104), obtained at the European Southern Observatory 3.6m telescope. These are the first mid-infrared spectra of metal-poor, low-mass stars. The spectrum of at least one of these, namely the extremely red, large-amplitude variable V1, shows direct evidence of circumstellar grains made of amorphous silicate.

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arXiv2005

Understanding Radio-Selected Thermal Sources in M 33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations

We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and radio images of 14 radio-selected objects in M 33. These objects are thought to represent the youngest phase of star cluster formation. We have detected the majority of cluster candidates in M 33 at all wavelengths. From the near-IR images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and stellar masses of 10^3-10^4 M_solar. We have generated spectral energy distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these SEDs, we have modeled the dust emission around these star clusters to determine the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters' local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV images are similar to within a factor of 2 or 3. These results suggest that eleven of the fourteen radio-selected objects are optically-visible young star clusters with a surrounding H II region, that two are background objects, possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region.

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arXiv2004

The Disk Atmospheres of Three Herbig Ae/Be Stars

We present infrared (IR) spectrophotometry ($R \simeq 180$) of three Herbig Ae/Be stars surrounded by possible protoplanetary disks: HD 150193, HD100546 and HD 179218. We construct a mid-IR spectral energy distributions (SED) for each object by using $7.6 - 13.2$ \micron HIFOGS spectra, 2.4 -- 45 \micron\ spectrophotometry from the {\it ISO} SWS, the 12, 25, 60, and 100 \micron\ photometric points from IRAS, and for HD 179218, photometric bolometric data points from the Mt. Lemmon Observing Facility. The SEDs are modeled by using an expanded version of the \citet{chigol97} two-layer, radiative and hydrostatic equilibrium, passive disk. This expanded version includes the emission from Mg-pure crystalline olivine (forsterite) grains in the disk surface layer. HD 150193 contains no crystals while HD 100546 and HD 179218 respectively show evidence of having crystalline silicates in the surface layers of their disks. We find that the inner region of HD100546 has a 37% higher crystalline-to-amorphous silicate ratio in its inner disk region ($\leq 5$ AU) compared to the outer disk region, while the inner disk region of HD 179218 has a 84% higher crystalline-to-amorphous silicate ratio in its inner disk region ($\leq 5$ AU) compared to the outer region. HD 150193 is best-fit by a small disk ($\sim 5$ AU in radius) while HD 100546 and HD 179218 are best fit by larger disks ($\sim 150$ AU in radius).

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arXiv2004

Chandra observations of the recurrent nova IM Nor

The recurrent nova IM Nor was observed twice in X-rays with Chandra ACIS-S, 1 and 6 months after the optical outburst. It was not detected in the first observation, with an upper limit on the X-ray luminosity in the 0.2-10 keV range L(x) < 4.8 x 10^(30) x (d/1 kpc)^2 erg s^(-1) (where d is the distance to the nova). Five months later, a hard X-ray source with L(x) = (1.4-2.5) x 10^(32) x (d/1 kpc)^2 erg s^(-1) was detected. The X-ray spectrum appears to be thermal, but we cannot rule out additional components due to unresolved emission lines. A blackbody component is likely to contribute to the observed spectrum, but it has bolometric luminosity L$_{\rm bol} = 2.5 x 10^(33) x (d/1 kpc)^2 erg s^(-1), therefore it is not sufficiently luminous to be due to a central white dwarf which is still burning hydrogen on the surface. An optical spectrum, taken 5 months post-outburst, indicates no intrinsic reddening of the ejecta. Therefore, we conclude that the shell had already become optically thin to supersoft X-rays, but nuclear burning had already turned off, or was in the process of turning off at this time. We discuss why this implies that recurrent novae, even the rare ones with long optical decays like IM Nor, indicating a large envelope mass, are not statistically significant as type Ia SN candidates.

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arXiv2004

Spitzer IRAC Observations of Star Formation in N159 in the LMC

We present observations of the giant HII region complex N159 in the LMC using IRAC on the {\it Spitzer Space Telescope}. One of the two objects previously identified as protostars in N159 has an SED consistent with classification as a Class I young stellar object (YSO) and the other is probably a Class I YSO as well, making these two stars the youngest stars known outside the Milky Way. We identify two other sources that may also be Class I YSOs. One component, N159AN, is completely hidden at optical wavelengths, but is very prominent in the infrared. The integrated luminosity of the entire complex is L $\approx 9\times10^6$L$_{\odot}$, consistent with the observed radio emission assuming a normal Galactic initial mass function (IMF). There is no evidence for a red supergiant population indicative of an older burst of star formation. The N159 complex is 50 pc in diameter, larger in physical size than typical HII regions in the Milky Way with comparable luminosity. We argue that all of the individual components are related in their star formation history. The morphology of the region is consistent with a wind blown bubble $\approx 1-2Myr-old that has initiated star formation now taking place at the rim. Other than its large physical size, star formation in N159 appears to be indistinguishable from star formation in the Milky Way.

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arXiv2004

Outflows from Luminous YSOs: An Infrared Polarimetric Study

e present Near Infrared imaging polarimetry of three regions of massive star formation, G$192.16 - 3.82$, Cepheus A, and W42. In W42 we have discovered a new bipolar nebula located at the far side of the HII region behind the visible cluster of exciting stars. The axis of this new nebula is aligned with the magnetic field threading the entire cluster region. Polarization in the bipolar outflow nebulosity associated with G192.16 is consistent with a single illuminating source, too faint to be detected at $2 \micron$. Polarization in the reflection nebulosity associated with Ceph A requires more than one illuminating source, although HW2 is clearly dominant. In all three objects, the magnetic field in the outflow at distances greater than $\sim 0.2$ pc is radial. In G192.16 the magnetic field geometry closer than $\sim 0.2$ pc to the embedded star appears chaotic. For G192.16 the outflow is not aligned with the surrounding magnetic field, which lies in the galactic plane. In Ceph A, the outflow axis could be interpreted as being aligned with the galactic plane, but the magnetic field threading the region is not. Only in the case of W42 is the magnetic field threading the HII region aligned with the mean field in the surrounding galactic plane.

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arXiv2002

The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 #2) in Outburst: The Discovery of Pulsations and a "Burst"

V1494 Aql (Nova Aql 1999 No. 2) was discovered on 2 December 1999. We obtained Chandra ACIS-I spectra on 15 April and 7 June 2000 which appear to show only emission lines. Our third observation, on 6 August, showed that its spectrum had evolved to that characteristic of a Super Soft X-ray Source. We then obtained Chandra LETG+HRC-S spectra on 28 September (8 ksec) and 1 October (17 ksec). We analyzed the X-ray light curve of our grating observations and found both a short time scale ``burst'' and oscillations. Neither of these phenomena have previously been seen in the light curve of a nova in outburst. The ``burst'' was a factor of 10 rise in X-ray counts near the middle of the second observation, and which lasted about 1000 sec; it exhibited at least two peaks, in addition to other structure. Our time series analysis of the combined 25 ksec observation shows a peak at 2500 s which is present in independent analyses of both the zeroth order image and the dispersed spectrum and is not present in similar analyses of grating data for HZ 43 and Sirius B. Further analyses of the V1494 Aql data find other periods present which implies that we are observing non-radial g+ modes from the pulsating, rekindled white dwarf.

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