A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars

We have performed a comprehensive study of the UV emission detected from AGB stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the NUV bandpass ($\lambda_{\rm eff} \sim 2310$z A), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the FUV bandpass ($\lambda_{\rm eff} \sim1528$ A). We find that NUV emission is correlated with optical to near infrared emission leading to higher detection fractions among the brightest, hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission, and possibly, the FUV emission are anti-correlated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.Comment: Accepted by ApJ; go spurs go

A New Radio Molecular Line Survey of Planetary Nebulae: HNC/HCN as a Diagnostic of Ultraviolet Irradiation

Certain planetary nebulae contain shells, filaments, or globules of cold gas and dust whose heating and chemistry are likely driven by UV and X-ray emission from their central stars and from wind-collision-generated shocks. We present the results of a survey of molecular line emission in the 88-236 GHz range from nine nearby (<1.5 kpc) planetary nebulae spanning a range of UV and X-ray luminosities, using the 30 m telescope of the Institut de Radioastronomie Millimetrique. Rotational transitions of thirteen molecules, including CO isotopologues and chemically important trace species, were observed and the results compared with and augmented by previous studies of molecular gas in PNe. Lines of the molecules HCO+, HNC, HCN, and CN, which were detected in most objects, represent new detections for five planetary nebulae in our study. Specifically, we present the first detections of 13CO (1-0, 2-1), HCO+, CN, HCN, and HNC in NGC 6445; HCO+ in BD+303639; 13CO (2-1), CN, HCN, and HNC in NGC 6853; and 13CO (2-1) and CN in NGC 6772. Flux ratios were analyzed to identify correlations between the central star and/or nebular UV and X-ray luminosities and the molecular chemistries of the nebulae. This analysis reveals a surprisingly robust dependence of the HNC/HCN line ratio on PN central star UV luminosity. There exists no such clear correlation between PN X-rays and various diagnostics of PN molecular chemistry. The correlation between HNC/HCN ratio and central star UV luminosity demonstrates the potential of molecular emission line studies of PNe for improving our understanding of the role that high-energy radiation plays in the heating and chemistry of photodissociation regions.Comment: 17 pages, 17 figures, 6 tables, accepted for publication in Astronomy & Astrophysic

Molecules in the transition disk orbiting T Cha

We seek to establish the presence and properties of gas in the circumstellar disk orbiting T Cha, a nearby (d~110 pc), relatively evolved (age ~5-7 Myr) yet actively accreting 1.5 Msun T Tauri star. We used the APEX 12 m radiotelescope to search for submillimeter molecular emission from the T Cha disk, and we reanalyzed archival XMM-Newton spectroscopy of T Cha to ascertain the intervening absorption due to disk gas along the line of sight to the star (N_H). We detected submillimeter rotational transitions of 12CO, 13CO, HCN, CN and HCO+ from the T Cha disk. The 12CO line appears to display a double-peaked line profile indicative of Keplerian rotation. Analysis of the CO emission line data indicates that the disk around T Cha has a mass (M_disk,H_2 = 80 M_earth) similar to, but more compact (R_disk, CO~80 AU) than, other nearby, evolved molecular disks (e.g. V4046 Sgr, TW Hya, MP Mus) in which cold molecular gas has been previously detected. The HCO+/13CO and HCN/13CO, line ratios measured for T Cha appear similar to those of other evolved circumstellar disks (i.e. TW Hya and V4046 Sgr), while the CN/13CO ratio appears somewhat weaker. Analysis of the XMM-Newton data shows that the atomic absorption $N_H$ toward T Cha is 1-2 orders of magnitude larger than toward the other nearby T Tauri with evolved disks. Furthermore, the ratio between atomic absorption and optical extinction N_H/A_V toward T Cha is higher than the typical value observed for the interstellar medium and young stellar objects in the Orion Nebula Cluster. This may suggest that the fraction of metals in the disk gas is higher than in the interstellar medium. Our results confirm that pre-main sequence stars older than ~5 Myr, when accreting, retain cold molecular disks, and that those relatively evolved disks display similar physical and chemical properties.Comment: Accepted for publication on A&

Serendipitous Chandra X-ray Detection of a Hot Bubble within the Planetary Nebula NGC 5315

We report the serendipitous detection of the planetary nebula NGC 5315 by the Chandra X-ray Observatory. The Chandra imaging spectroscopy results indicate that the X-rays from this PN, which harbors a Wolf-Rayet (WR) central star, emanate from a TX 2.5 × 106 K plasma generated via the same wind collisions that have cleared a compact ( 8000 AU radius) central cavity within the nebula. The inferred X-ray luminosity of NGC 5315 is 2.5 × 1032 erg s−1 (0.3-2.0 keV), placing this object among the most luminous such “hot bubble” X-ray sources yet detected within PNe. With the X-ray detection of NGC 5315, objects with WR-type central stars now constitute a clear majority – 2 – of known examples of diffuse X-ray sources among PNe; all such “hot bubble” PN X-ray sources display well-defined, quasi-continuous optical rims. We therefore assert that X-ray-luminous hot bubbles are characteristic of young PNe with large central star wind kinetic energies and closed bubble morphologies. However, the evidence at hand also suggests that processes such as wind and bubble temporal evolution, as well as heat conduction and/or mixing of hot bubble and nebular gas, ultimately govern the luminosity and temperature of superheated plasma within PNe

Mapping NGC 7027 in New Light: CO+^+ and HCO+^+ Emission Reveal Its Photon- and X-ray-Dominated Regions

The young and well-studied planetary nebula NGC 7027 harbors significant molecular gas that is irradiated by luminous, point-like UV (central star) and diffuse (shocked nebular) X-ray emission. This nebula represents an excellent subject to investigate the molecular chemistry and physical conditions within photon- and X-ray-dominated regions (PDRs and XDRs). As yet, the exact formation routes of CO$^+$ and HCO$^+$ in PN environments remain uncertain. Here, we present $\sim$2$"$ resolution maps of NGC 7027 in the irradiation tracers CO$^+$ and HCO$^+$, obtained with the IRAM NOEMA interferometer, along with SMA CO and HST 2.12~$\mu$m H$_2$ data for context. The CO$^+$ map constitutes the first interferometric map of this molecular ion in any PN. Comparison of CO$^+$ and HCO$^+$ maps reveal strikingly different emission morphologies, as well as a systematic spatial displacement between the two molecules; the regions of brightest HCO$^+$, found along the central waist of the nebula, are radially offset by $\sim$1$"$ ($\sim$900 au) outside the corresponding CO$^+$ emission peaks. The CO$^+$ emission furthermore precisely traces the inner boundaries of the nebula's PDR (as delineated by near-IR H$_2$ emission), suggesting that central star UV emission drives CO$^+$ formation. The displacement of HCO$^+$ radially outward with respect to CO$^+$ is indicative that dust-penetrating soft X-rays are responsible for enhancing the HCO$^+$ abundance in the surrounding molecular envelope, forming an XDR. These interferometric CO$^+$ and HCO$^+$ observations of NGC 7027 thus clearly establish the spatial distinction between the PDR and XDR formed (respectively) by intense UV and X-ray irradiation of molecular gas.Comment: 15 pages, 7 figures, 1 tabl

The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions

We present an overview of the initial results from the Chandra Planetary Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc of Earth, yielding 4 detections of diffuse X-ray emission and 9 detections of X-ray-luminous point sources at the central stars (CSPNe) of these objects. Combining these results with those obtained from Chandra archival data for all (14) other PNe within ~1.5 kpc that have been observed to date, we find an overall X-ray detection rate of ~70%. Roughly 50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing shocks formed by energetic wind collisions is detected in ~30%; five objects display both diffuse and point-like emission components. The presence of X-ray sources appears correlated with PN density structure, in that molecule-poor, elliptical nebulae are more likely to display X-ray emission (either point-like or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~100 kK) central star photospheres, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback. Most PNe detected as diffuse X-ray sources are elliptical nebulae that display a nested shell/halo structure and bright ansae; the diffuse X-ray emission regions are confined within inner, sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the timescale for strong shocks due to wind interactions in PNe.Comment: 41 pages, 6 figures; submitted to the Astronomical Journa

Inside-Out Evacuation of Transitional Protoplanetary Disks by the Magneto-Rotational Instability

How do T Tauri disks accrete? The magneto-rotational instability (MRI) supplies one means, but protoplanetary disk gas is typically too poorly ionized to be magnetically active. Here we show that the MRI can, in fact, explain observed accretion rates for the sub-class of T Tauri disks known as transitional systems. Transitional disks are swept clean of dust inside rim radii of ~10 AU. Stellar coronal X-rays ionize material in the disk rim, activating the MRI there. Gas flows from the rim to the star, at a rate limited by the depth to which X-rays ionize the rim wall. The wider the rim, the larger the surface area that the rim wall exposes to X-rays, and the greater the accretion rate. Interior to the rim, the MRI continues to transport gas; the MRI is sustained even at the disk midplane by super-keV X-rays that Compton scatter down from the disk surface. Accretion is therefore steady inside the rim. Blown out by radiation pressure, dust largely fails to accrete with gas. Contrary to what is usually assumed, ambipolar diffusion, not Ohmic dissipation, limits how much gas is MRI-active. We infer values for the transport parameter alpha on the order of 0.01 for GM Aur, TW Hyd, and DM Tau. Because the MRI can only afflict a finite radial column of gas at the rim, disk properties inside the rim are insensitive to those outside. Thus our picture provides one robust setting for planet-disk interaction: a protoplanet interior to the rim will interact with gas whose density, temperature, and transport properties are definite and decoupled from uncertain initial conditions. Our study also supplies half the answer to how disks dissipate: the inner disk drains from the inside out by the MRI, while the outer disk photoevaporates by stellar ultraviolet radiation.Comment: Accepted to Nature Physics June 7, 2007. The manuscript for publication is embargoed per Nature policy. This arxiv.org version contains more technical details and discussion, and is distributed with permission from the editors. 10 pages, 4 figure