A Herschel study of NGC 650 van Hoof, P. A. M.; Van de Steene, G. C.; Exter, K. M. ...
Astronomy and astrophysics (Berlin),
12/2013, Letnik:
560
Journal Article
Recenzirano
As part of the Herschel guaranteed time key project Mass loss of Evolved StarS (MESS) we have imaged a sample of planetary nebulae. In this paper we present the Photodetector Array Camera and ...Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) images of the classical bipolar planetary nebula NGC 650. We used these images to derive a temperature map of the dust. We also constructed a photoionization and dust radiative transfer model using the spectral synthesis code Cloudy. To constrain this model, we used the PACS and SPIRE fluxes and combined them with hitherto unpublished International Ultraviolet Explorer (IUE) and Spitzer InfraRed Spectrograph (IRS) spectra as well as various other data from the literature. A temperature map combined with a photoionization model were used to study various aspects of the central star, the nebula, and in particular the dust grains in the nebula. The central star parameters are determined to be Teff = 208 kK and L = 261 L⊙ assuming a distance of 1200 pc. The stellar temperature is much higher than previously published values. We confirm that the nebula is carbon-rich with a C/O ratio of 2.1. The nebular abundances are typical for a type IIa planetary nebula. With the photoionization model we determined that the grains in the ionized nebula are large (assuming single-sized grains, they would have a radius of 0.15 μm). Most likely these large grains were inherited from the asymptotic giant branch phase. The PACS 70/160 μm temperature map shows evidence of two radiation components heating the grains. The first component is direct emission from the central star, while the second component is diffuse emission from the ionized gas (mainly Lyα). We show that previous suggestions of a photo-dissociation region surrounding the ionized region are incorrect. The neutral material resides in dense clumps inside the ionized region. These may also harbor stochastically heated very small grains in addition to the large grains.
Detached circumstellar dust shells are detected around three carbon variables using Herschel-PACS. Two of them are already known on the basis of their thermal CO emission and two are visible as ...extensions in IRAS imaging data. By model fits to the new data sets, physical sizes, expansion timescales, dust temperatures, and more are deduced. A comparison with existing molecular CO material shows a high degree of correlation for TT Cyg and U Ant but a few distinct differences with other observables are also found.
Herschel PACS and SPIRE imaging of CW Leonis Ladjal, D.; Barlow, M. J.; Groenewegen, M. A. T. ...
Astronomy and astrophysics (Berlin),
07/2010, Letnik:
518, Številka:
1
Journal Article
Recenzirano
Herschel PACS and SPIRE images have been obtained over a 30' × 30' area around the well-known carbon star CW Leo (IRC +10 216). An extended structure is found in an incomplete arc of ~22' diameter, ...which is cospatial with the termination shock due to interaction with the interstellar medium (ISM) as defined by Sahai & Chronopoulos from ultraviolet GALEX images. Fluxes are derived in the 70, 160, 250, 350, and 550 μm bands in the region where the interaction with the ISM takes place, and this can be fitted with a modified black body with a temperature of 25 ± 3 K. Using the published proper motion and radial velocity for the star, we derive a heliocentric space motion of 25.1 km s-1. Using the PACS and SPIRE data and the analytical formula of the bow shock structure, we infer a de-projected standoff distance of the bow shock of R0 = (8.0 ± 0.3) × 1017 cm. We also derive a relative velocity of the star with respect to the ISM of (106.6 ± 8.7)/$\sqrt{n_{\rm ISM}}$ km s-1, where nISM is the number density of the local ISM.
A study of two post-common envelope binary systems Exter, K. M.; Pollacco, D. L.; Maxted, P. F. L. ...
Monthly Notices of the Royal Astronomical Society,
20/May , Letnik:
359, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present intermediate-resolution, phase-resolved spectra of the close-binary systems EC 11575−1845 and V664 Cas, the central star of the planetary nebula HFG 1. Both systems have a very rich ...emission-line spectrum originating from the face of the cool star, which is irradiated by the hot subdwarf star. The H i emission lines are wide, consistent with Stark broadening. These have a complex shape due to strong absorption near the line centre. We report on new spectroscopic orbits for both subdwarf stars, based on radial velocity measurements of their He ii lines. The orbits of the heated face of the cool companions were measured from narrow emission lines, and from the wide H i profiles; some differences were found between ions. At phases around 0, absorption lines from the photosphere of the secondary star of V664 Cas are seen. These are used to estimate a spectral type of mid-F to early K, and we report on their radial velocities. Synthetic spectra from the non-local thermodynamic equilibrium model atmospheres were used to fit features in the optical spectrum of the hot subdwarf star of EC 11575−1845, from which we estimate Teff= 105 000 ± 20 000 K, log g= 6.7 ± 0.3, log nHe/nH=−0.8 ± 0.1. Finally, we consider the most likely range of component masses for these two systems.
Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer ...parts of the nebula are recombining. From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H2 emission, which appears to be observational evidence that H2 forms on grain surfaces. We have developed a photoionization model of the nebula with the Cloudy code which we used to determine the physical conditions of the dust and investigate possible formation scenarios for the H2. We conclude that the most plausible scenario is that the H2 resides in high density knots which were formed after the recombination of the gas started when the central star entered the cooling track. Hydrodynamical instabilities due to the unusually low temperature of the recombining gas are proposed as a mechanism for forming the knots. H2 formation in the knots is expected to be substantial after the central star underwent a strong drop in luminosity about one to two thousand years ago, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots.
We present far-infrared and submillimetre spectra of three carbon-rich evolved objects, AFGL 2688, AFGL 618 and NGC 7027. The spectra were obtained with the SPIRE Fourier-transform spectrometer on ...board the Herschel Space Observatory, and cover wavelengths from 195–670 μm, a region of the electromagnetic spectrum hitherto difficult to study in detail. The far infrared spectra of these objects are rich and complex, and we measure over 150 lines in each object. Lines due to 18 different species are detected. We determine physical conditions from observations of the rotational lines of several molecules, and present initial large velocity gradient models for AFGL 618. We detect water in AFGL 2688 for the first time, and confirm its presence in AFGL 618 in both ortho and para forms. In addition, we report the detection of the J = 1–0 line of CH+ in NGC 7027.
Context: A planetary nebula (PN) candidate was discovered during FORS imaging of the Local Group dwarf galaxy Phoenix. Aims: We use this PN to complement abundances from red-giant stars. Methods: ...FORS spectroscopy was used to confirm the PN classification. Empirical methods and photoionization modeling were used to derive elemental abundances from the emission line fluxes and to characterize the central star. Results: For the elements deemed most reliable for measuring the metallicity of the interstellar medium (ISM) from which the PN formed, O/H ~ -0.46 and Ar/H ~ -1.03. O/H has lower measurement errors but greater uncertainties due to the unresolved issue of oxygen enrichment in the PN precursor star. Conclusions: Earlier than 2 Gyr ago (the lower limit of the derived age for the central star) the ISM had Z = 0.002-0.008, a range slightly more metal-rich than the one provided by stars. Comparing our PN-to-stellar values to surveys of other dwarf Local Group galaxies, Phoenix appears to be an outlier.
We report on the detection of anhydrous hydrochloric acid (hydrogen chlorine, HCl) in the carbon-rich star IRC +10216 using the spectroscopic facilities onboard the Herschel satellite. Lines from J = ...1-0 up to J = 7-6 have been detected. From the observed intensities, we conclude that HCl is produced in the innermost layers of the circumstellar envelope with an abundance relative to H2 of 5 × 10-8 and extends until the molecules reach its photodissociation zone. Upper limits to the column densities of AlH, MgH, CaH, CuH, KH, NaH, FeH, and other diatomic hydrides have also been obtained.
New data are presented of the post-common-envelope binary VW Pyx and its associated planetary nebula (PN), K 1-2. The central star is found to be a single-lined (and possibly double-lined) ...spectroscopic binary and is a good candidate for a system in which the main-sequence secondary star is more massive than the subdwarf primary star. The old PN has a high excitation level and a measured electron temperature, TO∼ 17 000 K for an assumed log ne= 2.7 cm−3. The so-called ‘jets’ are found to be lower-ionization regions with TN∼ 11 000 K and TO∼ 16 000 K with log ne∼ 2.5 cm−3 and with low, but fairly typical, PN abundances. We discuss possible histories for K 1-2 and the jets. However, there are still many unknowns in the study of accretion and jet formation for such PN binary central stars.
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