The Chandra Planetary Nebula Survey (ChanPlaNS). III. X-Ray Emission from the Central Stars of Planetary Nebulae
We present X-ray spectral analysis of 20 point-like X-ray sources detected in Chandra Planetary Nebula Survey observations of 59 planetary nebulae (PNe) in the solar neighborhood. Most of these 20 detections are associated with luminous central stars within relatively young, compact nebulae. The vast majority of these point-like X-ray-emitting sources at PN cores display relatively "hard" (>=0.5 keV) X-ray emission components that are unlikely to be due to photospheric emission from the hot central stars (CSPN). Instead, we demonstrate that these sources are well modeled by optically thin thermal plasmas. From the plasma properties, we identify two classes of CSPN X-ray emission: (1) high-temperature plasmas with X-ray luminosities, L X, that appear uncorrelated with the CSPN bolometric luminosity, L bol and (2) lower-temperature plasmas with L X/L bol ~ 10-7. We suggest these two classes correspond to the physical processes of magnetically active binary companions and self-shocking stellar winds, respectively. In many cases this conclusion is supported by corroborative multiwavelength evidence for the wind and binary properties of the PN central stars. By thus honing in on the origins of X-ray emission from PN central stars, we enhance the ability of CSPN X-ray sources to constrain models of PN shaping that invoke wind interactions and binarity.
Montez, R., Jr., Kastner, J. H., Balick, B., Behar, E., Blackman, E., Bujarrabal, V., Chu, Y.-H., Corradi, R. L. M., De Marco, O., Frank, A., Freeman, M., Frew, D. J., Guerrero, M. A., Jones, D., Lopez, J. A., Miszalski, B., Nordhaus, J., Parker, Q. A., Sahai, R., Sandin, C., Schonberner, D., Soker, N., Sokoloski, J. L., Steffen, M., Toalá, J. A., Ueta, T., Villaver, E., and Zijlstra, A.
Constraints on the Surface Magnetic Fields and Age of a Cool Hypergiant: XMM-Newton X-Ray Observations of VY CMa
The complex circumstellar ejecta of highly evolved, cool hypergiants are indicative of multiple, asymmetric mass-loss events. To explore whether such episodic, non-isotropic mass loss may be driven by surface magnetic activity, we have observed the archetypical cool hypergiant VY CMa with the XMM-Newton X-ray satellite observatory. The hypergiant itself is not detected in these observations. From the upper limit on the X-ray flux from VY CMa at the time of our observations (F X, UL ≈ 8 × 10–14 erg cm–2 s–1, corresponding to log LX /L bol <= –8), we estimate an average surface magnetic field strength fB <= 2 × 10–3 G (where f is the filling factor of magnetically active surface regions). These X-ray results for VY CMa represent the most stringent constraints to date on the magnetic field strength near the surface of a hypergiant. VY CMa's mass loss is episodic, however, and the hypergiant may have been in a state of low surface magnetic activity during the XMM observations. The XMM observations also yield detections of more than 100 X-ray sources within ~15' of VY CMa, roughly 50 of which have near-infrared counterparts. Analysis of X-ray hardness ratios and IR colors indicates that some of these field sources may be young, late-type stars associated with VY CMa, its adjacent molecular cloud complex, and the young cluster NGC 2362. Further study of the VY CMa field is warranted, given the potential to ascertain the evolutionary timescale of this enigmatic, massive star.
Montez, Rodolfo, Jr., Kastner, Joel H., Humphreys, Roberta M., Turok, Rebecca L., and Davidson, Kris
First Detection of Ultraviolet Emission from a Detached Dust Shell: Galaxy Evolution Explorer Observations of the Carbon Asymptotic Giant Branch Star U Hya
We present the discovery of an extended ring of ultraviolet (UV) emission surrounding the asymptotic giant branch (AGB) star U Hya in archival observations performed by the Galaxy Evolution Explorer. This is the third discovery of extended UV emission from a carbon AGB star and the first from an AGB star with a detached shell. From imaging and photometric analysis of the FUV and NUV images, we determined that the UV ring has a radius of ~110'', thus indicating that the emitting material is likely associated with the detached shell seen in the infrared. We find that scattering of the central point source of NUV and FUV emission by the dust shell is negligible. Moreover, we find that scattering of the interstellar radiation field by the dust shell can contribute at most ~10% of the FUV flux. Morphological and photometric evidence suggests that shocks caused by the star's motion through space and, possibly, shock-excited H2 molecules are the most likely origins of the UV flux. In contrast to previous examples of extended UV emission from AGB stars, the extended UV emission from U Hya does not show a bow-shock-like structure, which is consistent with a lower space velocity and lower interstellar medium density. This suggests the detached dust shell is the source of the UV-emitting material and can be used to better understand the formation of detached shells.
Sanchez, Enmanuel, Montez, Rodolfo, Jr., Ramstedt, Sofia, and Stassun, Keivan G.
The Chandra Planetary Nebula Survey (CHANPLANS). II. X-Ray Emission from Compact Planetary Nebulae
We present results from the most recent set of observations obtained as part of the Chandra X-ray observatory Planetary Nebula Survey (CHANPLANS), the first comprehensive X-ray survey of planetary nebulae (PNe) in the solar neighborhood (i.e., within ~1.5 kpc of the Sun). The survey is designed to place constraints on the frequency of appearance and range of X-ray spectral characteristics of X-ray-emitting PN central stars and the evolutionary timescales of wind-shock-heated bubbles within PNe. CHANPLANS began with a combined Cycle 12 and archive Chandra survey of 35 PNe. CHANPLANS continued via a Chandra Cycle 14 Large Program which targeted all (24) remaining known compact (R neb <~ 0.4 pc), young PNe that lie within ~1.5 kpc. Results from these Cycle 14 observations include first-time X-ray detections of hot bubbles within NGC 1501, 3918, 6153, and 6369, and point sources in HbDs 1, NGC 6337, and Sp 1. The addition of the Cycle 14 results brings the overall CHANPLANS diffuse X-ray detection rate to ~27% and the point source detection rate to ~36%. It has become clearer that diffuse X-ray emission is associated with young (lsim 5 × 103 yr), and likewise compact (R neb <~ 0.15 pc), PNe with closed structures and high central electron densities (ne >~ 1000 cm-3), and is rarely associated with PNe that show H2 emission and/or pronounced butterfly structures. Hb 5 is one such exception of a PN with a butterfly structure that hosts diffuse X-ray emission. Additionally, two of the five new diffuse X-ray detections (NGC 1501 and NGC 6369) host [WR]-type central stars, supporting the hypothesis that PNe with central stars of [WR]-type are likely to display diffuse X-ray emission.
Freeman, M., Montez, R., Jr., Kastner, J. H., Balick, B., Frew, D. J., Jones, D., Miszalski, B., Sahai, R., Blackman, E., Chu, Y.-H., De Marco, O., Frank, A., Guerrero, M. A., Lopez, J. A., Zijlstra, A., Bujarrabal, V., Corradi, R. L. M., Nordhaus, J., Parker, Q. A., Sandin, C., Schönberner, D., Soker, N., Sokoloski, J. L., Steffen, M., Toalá, J. A., Ueta, T., and Villaver, E.
The wonderful complexity of the Mira AB system
We have mapped the 12CO(3-2) line emission around the Mira AB system at 0.̋5 resolution using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO map shows amazing complexity. The circumstellar gas has been shaped by different dynamical actors during the evolution of the system, and several morphological components can be identified. The companion is marginally resolved in continuum emission and is currently at 0.̋487 ± 0.̋006 separation. In the main line component, centered on the stellar velocity, spiral arcs around Mira A are found. The spiral appears to be relatively flat and oriented in the orbital plane. An accretion wake behind the companion is clearly visible, and the projected arc separation is about 5''. In the blue wing of the line emission, offset from the main line, several large (~5-10'') opposing arcs are found. We tentatively suggest that this structure is created by the wind of Mira B blowing a bubble in the expanding envelope of Mira A.
Appendix A is available in electronic form at http://www.aanda.org
Ramstedt, S., Mohamed, S., Vlemmings, W. H. T., Maercker, M., Montez, R., Baudry, A., De Beck, E., Lindqvist, M., Olofsson, H., Humphreys, E. M. L., Jorissen, A., Kerschbaum, F., Mayer, A., Wittkowski, M., Cox, N. L. J., Lagadec, E., Leal-Ferreira, M. L., Paladini, C., Pérez-Sánchez, A., and Sacuto, S.
Herschel Planetary Nebula Survey (HerPlaNS). First detection of OH+ in planetary nebulae
Aims: We report the first detections of OH+ emission in planetary nebulae (PNe).
Methods: As part of an imaging and spectroscopy survey of 11 PNe in the far-IR using the PACS and SPIRE instruments aboard the Herschel Space Observatory, we performed a line survey in these PNe over the entire spectral range between 51μm and 672μm to look for new detections.
Results: The rotational emission lines of OH+ at 152.99, 290.20, 308.48, and 329.77μm were detected in the spectra of three planetary nebulae:
Conclusions: In PNe, the OH+ rotational line emission appears to be produced in the photodissociation region (PDR) in these objects. The emission of OH+ is observed only in PNe with hot central stars (Teff> 100 000 K), suggesting that high-energy photons may play a role in OH+ formation and its line excitation in these objects, as seems to be the case for ultraluminous galaxies.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Aleman, I., Ueta, T., Ladjal, D., Exter, K. M., Kastner, J. H., Montez, R., Tielens, A. G. G. M., Chu, Y.-H., Izumiura, H., McDonald, I., Sahai, R., Siódmiak, N., Szczerba, R., van Hoof, P. A. M., Villaver, E., Vlemmings, W., Wittkowski, M., and Zijlstra, A. A.
The Herschel Planetary Nebula Survey (HerPlaNS). I. Data overview and analysis demonstration with NGC 6781
Context. This is the first of a series of investigations into far-IR characteristics of 11 planetary nebulae (PNe) under the Herschel Space Observatory open time 1 program, Herschel Planetary Nebula Survey (HerPlaNS).
Aims: Using the HerPlaNS data set, we look into the PN energetics and variations of the physical conditions within the target nebulae. In the present work, we provide an overview of the survey, data acquisition and processing, and resulting data products.
Methods: We performed (1) PACS/SPIRE broadband imaging to determine the spatial distribution of the cold dust component in the target PNe and (2) PACS/SPIRE spectral-energy-distribution and line spectroscopy to determine the spatial distribution of the gas component in the target PNe.
Results: For the case of
Conclusions: The HerPlaNS data provide various diagnostics for both the dust and gas components in a spatially resolved manner. In the forthcoming papers of the HerPlaNS series we will explore the HerPlaNS data set fully for the entire sample of 11 PNe.
Herschel is an ESA Space Observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Table 2 and appendices are available in electronic form at http://www.aanda.org
Ueta, T., Ladjal, D., Exter, K. M., Otsuka, M., Szczerba, R., Siódmiak, N., Aleman, I., van Hoof, P. A. M., Kastner, J. H., Montez, R., McDonald, I., Wittkowski, M., Sandin, C., Ramstedt, S., De Marco, O., Villaver, E., Chu, Y.-H., Vlemmings, W., Izumiura, H., Sahai, R., Lopez, J. A., Balick, B., Zijlstra, A., Tielens, A. G. G. M., Rattray, R. E., Behar, E., Blackman, E. G., Hebden, K., Hora, J. L., Murakawa, K., Nordhaus, J., Nordon, R., and Yamamura, I.
Molecules in the transition disk orbiting T Chamaeleontis
Aims: We seek to establish the presence and properties of gas in the circumstellar disk orbiting
Methods: We used the Atacama Pathfinder Experiment (APEX) 12 m radiotelescope to search for submillimeter molecular emission from the T Cha disk, and we reanalyzed archival XMM-Newton imaging spectroscopy of T Cha to ascertain the intervening absorption due to disk gas along the line of sight to the star (NH).
Results: We detected submillimeter rotational transitions of 12CO, 13CO, HCN, CN, and HCO+ from the T Cha disk. The 12CO line (and possibly the 13CO line) appears to display a double-peaked line profile indicative of Keplerian rotation; hence, these molecular line observations constitute the first direct demonstration of the presence of cold molecular gas orbiting T Cha. Analysis of the CO emission line data indicates that the disk around T Cha has a mass (Mdisk,H2 = 80 M⊕) similar to, but more compact (Rdisk,CO ~ 80 AU) than other nearby, evolved molecular disks (e.g.,
Conclusions: Our results confirm that pre-main-sequence stars older than ~5 Myr retain cold molecular disks when accreting, and that those relatively evolved disks display similar physical and chemical properties.
Based on submillimeter and X-ray observations. Submillimeter observations have been collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, with the Atacama Pathfinder Experiment APEX (Prog. ID 088.C-0441 and E-089.C-0518A). X-ray archival observations used in this paper have been obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.
Sacco, G. G., Kastner, J. H., Forveille, T., Principe, D., Montez, R., Zuckerman, B., and Hily-Blant, P.
Serendipitous Detection of X-Ray Emission from the Hot Born-again Central Star of the Planetary Nebula K 1-16
We report the serendipitous detection of point-like X-ray emission from the hot, PG1159-type central star of the planetary nebula (CSPN) K 1-16 by the XMM-Newton and Chandra X-Ray Observatories. The CSPN lies superimposed on a galaxy cluster that includes an X-ray-bright quasar, but we have successfully isolated the CSPN X-ray emission from the strong diffuse background contributed by the quasar and intracluster gas. We have modeled the XMM-Newton and Chandra X-ray data, taking advantage of the contrasting detection efficiencies of the two observatories to better constrain the low-energy spectral response of Chandra's Advanced CCD Imaging Spectrometer. We find that the CSPN X-ray spectrum is well characterized by the combination of a non-local thermodynamic equilibrium model atmosphere with T sstarf ~ 135 kK and a carbon-rich, optically thin thermal plasma with T X ~ 1 MK. These results for X-ray emission from the K 1-16 CSPN, combined with those obtained for other PG1159-type objects, lend support to the "born-again" scenario for Wolf-Rayet and PG1159 CSPNe, wherein a late helium shell flash dredges up carbon-rich intershell material and ejects this material into the circumstellar environment.
Montez, Rodolfo, Jr. and Kastner, Joel H.
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 four detections of diffuse X-ray emission and nine 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% for the 35 sample objects. Roughly 50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing shocks—in most cases, "hot bubbles"—formed by energetic wind collisions is detected in ~30%; five objects display both diffuse and point-like emission components. The presence (or absence) 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 point-like CSPNe X-ray sources display X-ray spectra that are harder than expected from hot (~100 kK) central stars emitting as simple blackbodies; the lone apparent exception is the central star of the Dumbbell nebula, NGC 6853. These hard X-ray excesses may suggest 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 <~ 5 × 103 yr, placing firm constraints on the timescale for strong shocks due to wind interactions in PNe. The high-energy emission arising in such wind shocks may contribute to the high excitation states of certain archetypical "hot bubble" nebulae (e.g., NGC 2392, 3242, 6826, and 7009).
Kastner, J. H., Montez, R., Jr., Balick, B., Frew, D. J., Miszalski, B., Sahai, R., Blackman, E., Chu, Y.-H., De Marco, O., Frank, A., Guerrero, M. A., Lopez, J. A., Rapson, V., Zijlstra, A., Behar, E., Bujarrabal, V., Corradi, R. L. M., Nordhaus, J., Parker, Q. A., Sandin, C., Schönberner, D., Soker, N., Sokoloski, J. L., Steffen, M., Ueta, T., and Villaver, E.
Searching for X-ray emission from AGB stars
Context. Magnetic fields have been measured around asymptotic giant branch (AGB) stars of all chemical types using maser polarization observations. If present, a large-scale magnetic field would lead to X-ray emission, which should be observable using current X-ray observatories.
Aims: The aim is to search the archival data for AGB stars that are intrinsic X-ray emitters.
Methods: We have searched the ROSAT, CXO, and XMM-Newton archives for serendipitous X-ray observations of a sample of ~500 AGB stars. We specifically searched for the AGB stars detected with GALEX. The data is calibrated, analyzed and the X-ray luminosities and temperatures are estimated as functions of the circumstellar absorption.
Results: We identify 13 AGB stars as having either serendipitous or targeted observations in the X-ray data archives, however for a majority of the sources the detailed analysis show that the detections are questionable. Two new sources are detected by ROSAT: T Dra and R UMa. The spectral analysis suggests that the emission associated with these sources could be due to coronal activity or interaction across a binary system.
Conclusions: Further observations of the detected sources are necessary to clearly determine the origin of the X-ray emission. Moreover, additional objects should be subject to targeted X-ray observations in order to achieve better constraints for the magnetic fields around AGB stars.
Appendices are available in electronic form at http://www.aanda.org
Ramstedt, S., Montez, R., Kastner, J., and Vlemmings, W. H. T.
Examining the Radio-loud/Radio-quiet Dichotomy with New Chandra and VLA Observations of 13 UGC Galaxies
We present the results from new ~15 ks Chandra-ACIS and 4.9 GHz Very Large Array (VLA) observations of 13 galaxies hosting low-luminosity active galactic nuclei (AGNs). This completes the multiwavelength study of a sample of 51 nearby early-type galaxies described in Capetti & Balmaverde and Balmaverde & Capetti. The aim of the three previous papers was to explore the connection between the host galaxies and AGN activity in a radio-selected sample. We detect nuclear X-ray emission in eight sources and radio emission in all but one (viz., UGC 6985). The new VLA observations improve the spatial resolution by a factor of 10: the presence of nuclear radio sources in 12 of the 13 galaxies confirms their AGN nature. As previously indicated, the behavior of the X-ray and radio emission in these sources depends strongly on the form of their optical surface brightness profiles derived from Hubble Space Telescope imaging, i.e., on their classification as "core," "power-law," or "intermediate" galaxies. With more than twice the number of "power-law" and "intermediate" galaxies compared to previous work, we confirm with a much higher statistical significance that these galaxies lie well above the radio-X-ray correlation established in Fanaroff-Riley type I radio galaxies and the low-luminosity "core" galaxies. This result highlights the fact that the "radio-loud/radio-quiet" dichotomy is a function of the host galaxy's optical surface brightness profile. We present radio-optical-X-ray spectral indices for all 51 sample galaxies. Survival statistics point to significant differences in the radio-to-optical and radio-to-X-ray spectral indices between the "core" and "power-law" galaxies (Gehan's Generalized Wilcoxon test probability p for the two classes being statistically similar is <10-5), but not in the optical-to-X-ray spectral indices (p = 0.25). Therefore, the primary difference between the "core" and "power-law" galaxies is in their ability to launch powerful radio outflows. This result is consistent with the hypothesis of different formation processes and evolution histories in "core" and "power-law" galaxies: major mergers are likely to have created "core" galaxies, while minor mergers were instrumental in the creation of "power-law" galaxies.
Kharb, P., Capetti, A., Axon, D. J., Chiaberge, M., Grandi, P., Robinson, A., Giovannini, G., Balmaverde, B., Macchetto, D., and Montez, R.
2M1155-79 (= T Chamaeleontis B): A Low-mass, Wide-separation Companion to the nearby, "Old" T Tauri Star T Chamaeleontis
The early-K star T Cha, a member of the nearby (D ≈ 100 pc) epsilon Cha Association, is a relatively "old" (age ~ 7 Myr) T Tauri star that is still sporadically accreting from an orbiting disk whose inner regions are now evidently being cleared by a close, substellar companion. We report the identification, via analysis of proper motions, serendipitous X-ray imaging spectroscopy, and follow-up optical spectroscopy, of a new member of the epsilon Cha Association that is very likely a low-mass companion to T Cha at a projected separation of ~38 kAU. The combined X-ray and optical spectroscopy data indicate that the companion, T Cha B (= 2M1155-79), is a weak-lined T Tauri star (wTTS) of spectral type M3 and age <~ 10 Myr. The serendipitous X-ray (XMM-Newton) observation of T Cha B, which targeted T Cha, also yields serendipitous detections of two background wTTS in the Chamaeleon cloud complex, including one newly discovered, low-mass member of the Cha cloud pre-main-sequence (pre-MS) population. T Cha becomes the third prominent example of a nearby, "old" yet still actively accreting, K-type pre-MS star/disk system (the others being TW Hya and V4046 Sgr) to feature a low-mass companion at very large (12-40 kAU) separation, suggesting that such wide-separation companions may affect the conditions and timescales for planet formation around solar-mass stars.
Kastner, Joel H., Thompson, Emily A., Montez, Rodolfo, Murphy, Simon J., Bessell, Michael S., and Sacco, Giuseppe Germano
GSC 07396-00759 = V4046 Sgr C[D]: A Wide-separation Companion to the Close T Tauri Binary System V4046 Sgr AB
We explore the possibility that GSC 07396-00759 (spectral type M1e) is a widely separated (~2farcm82, or projected separation ~12,350 AU) companion to the "old" (age ~12 Myr) classical T Tauri binary system V4046 Sgr AB, as suggested by the proximity and similar space motions of the two systems. If the two systems are equidistant and coeval, then GSC 07396-00759, like V4046 Sgr AB, must be a spectroscopic binary with nearly equal-mass components, and V4046 Sgr must be at least ~8 Myr old. Analysis of a serendipitous Chandra X-ray grating spectrum and light curve as well as XMM-Newton light curves and CCD spectra of GSC 07396-00759 obtained during long exposures targeting V4046 Sgr AB reveals a relatively hard (TX ~ 107 K) X-ray spectrum, strong flaring, and relatively low-density plasma. These X-ray characteristics of GCS 07396-00759 are indicative of a high level of coronal activity, consistent with its apparent weak-lined T Tauri star status. Interactions between V4046 Sgr AB and GCS 07396-00759 when the two systems were more closely bound may be responsible for (1) their dissolution ~106 yr ago, (2) the present tight, circular orbit of V4046 Sgr AB, and (3) the persistence of the gaseous circumbinary disk still orbiting V4046 Sgr AB.
Kastner, J. H., Sacco, G. G., Montez, R., Huenemoerder, D. P., Shi, H., Alecian, E., Argiroffi, C., Audard, M., Bouvier, J., Damiani, F., Donati, J.-F., Gregory, S. G., Güdel, M., Hussain, G. A. J., Maggio, A., and Montmerle, T.
X-ray Emission from the Binary Central Stars of the Planetary Nebulae HFG 1, DS 1, and LoTr 5
Close binary systems undergoing mass transfer or common envelope interactions can account for the morphological properties of some planetary nebulae. The search for close binary companions in planetary nebulae is hindered by the difficulty of detecting cool, late-type, main-sequence companions in binary systems with hot pre-white-dwarf primaries. However, models of binary planetary nebula progenitor systems predict that mass accretion or tidal interactions can induce rapid rotation in the companion, leading to X-ray-emitting coronae. To test such models, we have searched for, and detected, X-ray emission from three binary central stars within planetary nebulae: the post-common envelope close binaries in HFG 1 and DS 1 consisting of O-type subdwarfs with late-type, main-sequence companions and the binary system in LoTr 5 consisting of O-type subdwarf and rapidly rotating, late-type giant companion. The X-ray emission in each case is best characterized by spectral models consisting of two optically thin thermal plasma components with characteristic temperatures of ~10 MK and 15-40 MK and total X-ray luminosities ~1030 erg s-1. We consider the possible origin of the X-ray emission from these binary systems and conclude that the most likely origin is, in each case, a corona around the late-type companion, as predicted by models of interacting binaries.
Montez, Rodolfo, Jr., De Marco, Orsola, Kastner, Joel H., and Chu, You-Hua
Chandra X-ray Detection of the Enigmatic Field Star BP Psc
BP Psc is a remarkable emission-line field star that is orbited by a dusty disk and drives a parsec-scale system of jets. We report the detection by the Chandra X-ray Observatory of a weak X-ray point source coincident with the centroids of optical/IR and submillimeter continuum emission at BP Psc. As the star's photosphere is obscured throughout the visible and near-infrared, the Chandra X-ray source likely represents the first detection of BP Psc itself. The X-rays most likely originate with magnetic activity at BP Psc and hence can be attributed either to a stellar corona or to star-disk interactions. The log of the ratio of X-ray to bolometric luminosity, log(LX /L bol), lies in the range -5.8 to -4.2. This is smaller than log(LX /L bol) ratios typical of low-mass, pre-main sequence stars, but is well within the log(LX /L bol) range observed for rapidly rotating (FK Com-type) G giant stars. Hence, the Chandra results favor an exotic model wherein the disk/jet system of BP Psc is the result of its very recently engulfing a companion star or a giant planet, as the primary star ascended the giant branch.
Kastner, Joel H., Montez, Rodolfo, Jr., Rodriguez, David, Grosso, Nicolas, Zuckerman, B., Perrin, Marshall D., Forveille, Thierry, and Graham, James R.
XMM-Newton Detection of a Transient X-ray Source in the Vicinity of V838 Monocerotis
We report the XMM-Newton/EPIC detection in 2008 March of a luminous (LX ~ 1032-33 erg s-1), variable X-ray source in the vicinity (within ~6'') of the enigmatic star V838 Mon, which underwent a spectacular outburst in early 2002. Spectral modeling of the XMM-Newton X-ray source indicates the presence of two plasma components with characteristic temperatures of TX ~ 2 × 106 K and ~1.5 × 107 K, attenuated by an absorbing column (NH ~ 4 × 1021 cm-2) that is consistent with the visual extinction measured toward V838 Mon (AV ~ 2). No such luminous source was detected in the immediate vicinity of V838 Mon in Chandra/ACIS-S observations obtained about one year after outburst or, most recently, in 2010 January. The two XMM -Newton source spectral components appear to be marginally spatially resolved, with the spatial centroid of the hard component lying closer to (within ~2'' of) the position of V838 Mon than the broadband source or the soft source component; however, if there are two sources at or near V838 Mon, the Chandra nondetections would imply that both of them are variable. An alternative is that there is a single variable source, and that the apparent spatial separation may actually be due to photon-counting statistics or is perhaps instrumental in origin. We consider constraints placed by the X-ray detection and nondetections on a stellar merger model for the 2002 V838 Mon outburst, in which the spun-up merger product drives a powerful magnetic dynamo. Alternatively, the transient behavior of the X-ray source could indicate that the X-rays arose as a consequence of an interaction between V838 Mon's ejecta and its early-type (B3 V) companion.
Antonini, Fabio, Montez, Rodolfo, Jr., Kastner, Joel H., Bond, Howard E., Soker, Noam, Tylenda, Romuald, Starrfield, Sumner, and Behar, Ehud
Chandra Detection of Extended X-ray Emission from the Recurrent Nova RS Ophiuchi
Radio, infrared, and optical observations of the 2006 eruption of the symbiotic recurrent nova RS Ophiuchi showed that the explosion produced non-spherical ejecta. Some of this ejected material was in the form of bipolar jets to the east and west of the central source. Here we describe X-ray observations taken with the Chandra X-ray Observatory one and a half years after the beginning of the outburst that reveal narrow, extended structure with a position angle of approximately 300° (east of north). Although the orientation of the extended feature in the X-ray image is consistent with the readout direction of the CCD detector, extensive testing suggests that the feature is not an artifact. Assuming it is not an instrumental effect, the extended X-ray structure shows hot plasma stretching more than 1900 AU from the central binary (taking a distance of 1.6 kpc). The X-ray emission is elongated in the northwest direction—in line with the extended infrared emission and some minor features in the published radio image. It is less consistent with the orientation of the radio jets and the main bipolar optical structure. Most of the photons in the extended X-ray structure have energies of less than 0.8 keV. If the extended X-ray feature was produced when the nova explosion occurred, then its 1farcs2 length as of 2007 August implies that it expanded at an average rate of more than 2 mas day-1, which corresponds to a flow speed of greater than 6000 km s-1 (days/1.6 kpc) in the plane of the sky. This expansion rate is similar to the earliest measured expansion rates for the radio jets.
Luna, G. J. M., Montez, R., Sokoloski, J. L., Mukai, K., and Kastner, J. H.
Serendipitous XMM-Newton Detection of X-Ray Emission from the Bipolar Planetary Nebula Hb 5
We report the serendipitous detection by the XMM-Newton X-ray Observatory of an X-ray source at the position of the Type I (He- and N-rich) bipolar planetary nebula (PN) Hb 5. The Hb 5 X-ray source appears marginally resolved. While the small number of total counts (~170) and significant off-axis angle of the X-ray source (~7farcm8) precludes a definitive spatial analysis, the morphology of the X-ray emission appears to trace the brightest features seen in optical images of Hb 5. The X-ray spectrum is indicative of a thermal plasma at a temperature between 2.4 and 3.7 MK and appears to display strong Neon emission. The inferred X-ray luminosity is LX = 1.5 × 1032ergs-1. These results suggest that the detected X-ray emission is dominated by shock-heated gas in the bipolar nebula, although we cannot rule out the presence of a pointlike component at the position of the central star. The implications for and correspondence with current models of shock-heated gas in PNe is discussed.
Montez, Rodolfo, Jr., Kastner, Joel H., Balick, Bruce, and Frank, Adam
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 (W-R) central star, emanate from a TX~2.5×106 K plasma generated via the same wind-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 ergs 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 W-R-type central stars now constitute a clear majority 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.
Kastner, Joel H., Montez, Rodolfo, Jr., Balick, Bruce, and De Marco, Orsola
X-Ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-Ray Observatory observations of the planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type Wolf-Rayet central stars that are currently driving fast (~1000 km s-1), massive winds into denser, slow-moving (~10 km s-1) material ejected during recently terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these observations provide key tests of models of wind-wind interactions in PNs. In NGC 40, we detect faint, diffuse X-ray emission distributed within a partial annulus that lies nested within a ~40" diameter ring of nebulosity observed in optical and near-infrared images. Hen 2-99 is not detected. The inferred X-ray temperature (TX~106 K) and luminosity (LX~2×1030 ergs s-1) of NGC 40 are the lowest measured thus far for any PN displaying diffuse X-ray emission. These results, combined with the ringlike morphology of the X-ray emission from NGC 40, suggest that its X-ray emission arises from a ``hot bubble'' that is highly evolved and is generated by a shocked, quasi-spherical fast wind from the central star, as opposed to AGB or post-AGB jet activity. In contrast, the lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet to enter a phase of strong wind-wind shocks.
Montez, Rodolfo, Jr., Kastner, Joel H., De Marco, Orsola, and Soker, Noam
Light and Color Curves of Six Field RR Lyrae Variable Stars
Day et al. 2002 published in Publications of the Astronomical Society of the Pacific
We present light curves in the B, V, and I passbands for six field RR Lyrae stars. We derive the stars' intensity-mean magnitudes, light amplitudes, and colors at minimum light. We compare the latter with measurements from other variables and discuss the use of (V-I) color at minimum light as a tool for measuring interstellar reddening. From 15 stars, we find the mean dereddened RR Lyrae color at minimum light to be (V-I)0=0.57 mag. The rms scatter of 0.025 mag indicates the precision of this value over a wide range of metallicities and pulsation periods. This research was conducted in part during the 2001 Research Experiences for Undergraduates (REU) and Prácticas de Investigación en Astronomía (PIA) Programs at Cerro Tololo Inter-American Observatory (CTIO).
Day, Alicia S., Layden, Andrew C., Hoard, D. W., Brammer, Gabriel, Cooksey, Kathy, Cuadra, Jorge, Holmes, Shadrian, Labbe, Erika, and Montez, Rodolfo, Jr.
Kinematic and Morphological Modeling of the Bipolar Nebula Sa 2-237
We present [O III] 500.7 nm and Hα+[N II] images and long-slit, high-resolution echelle spectra in the same spectral regions of Sa 2-237, a possible bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent with a narrow waist and strong point symmetry around the central object, indicating its likely binary nature. The long-slit spectra were taken over the long axis of the nebula and show a distinct 8-shaped pattern in the velocity-space plot with a maximum projected outflow velocity of vexp=106 km s-1, both typical of expanding bipolar planetary nebulae. By model fitting the shape and spectrum of the nebula simultaneously, we derive the inclination of the long axis to be 70° and the maximum space velocity of expansion to be <=308 km s-1. Due to asymmetries in the velocity, we adopt a new value for the system's heliocentric radial velocity of -30 km s-1. We use the IRAS and 21 cm radio fluxes, the energy distribution, and the projected size of Sa 2-237 to estimate its distance to be 2.1+/-0.37 kpc. At this distance, Sa 2-237 has a luminosity of 340 Lsolar, a size of 0.37 pc, and-assuming a constant expansion velocity-a nebular age of 624 years. The above radial velocity and distance place Sa 2-237 in the disk of the Galaxy at z=255 pc, albeit with somewhat peculiar kinematics. Based on observations collected at the European Southern Observatory, Chile.
Schwarz, Hugo E., Corradi, Romano R. L., and Montez, Rodolfo, Jr.