Spitzer IRS Spectroscopy of High-Mass Precursors of Planetary Nebulae (original) (raw)
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Spitzer mid-infrared spectroscopic observations of planetary nebulae
Monthly Notices of the Royal Astronomical Society, 2016
We present Spitzer Space Telescope archival mid-infrared (mid-IR) spectroscopy of a sample of eleven planetary nebulae (PNe). The observations, acquired with the Spitzer Infrared Spectrograph (IRS), cover the spectral range 5.2-14.5 µm that includes the H 2 0-0 S(2) to S(7) rotational emission lines. This wavelength coverage has allowed us to derive the Boltzmann distribution and calculate the H 2 rotational excitation temperature (T ex). The derived excitation temperatures have consistent values ≃900±70 K for different sources despite their different structural components. We also report the detection of mid-IR ionic lines of [Ar III], [S IV], and [Ne II] in most objects, and polycyclic aromatic hydrocarbon (PAH) features in a few cases. The decline of the [Ar III]/[Ne II] line ratio with the stellar effective temperature can be explained either by a true neon enrichment or by high density circumstellar regions of PNe that presumably descend from higher mass progenitor stars.
The Astrophysical Journal, 1999
Based on ISO short-wavelength spectrograph (SWS) ] long-wavelength spectrograph (LWS) observations, IRAS 16594[4656 is identiÐed as a new member of the rare class of C-rich protoÈplanetary nebulae with the 21 km dust feature in emission. In addition, the major polycyclic aromatic hydrocarbon (PAH) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 km are detected, together with unusually strong features at 12.6 and 13.4 km, very rarely observed, attributed to PAH species with a high degree of hydrogenation. The shapes and relative intensities of the various features found are consistent with a mixed population of transient decomposition products of large hydrogenated amorphous carbonaceous (HAC) grains, consisting of fullerenes with di †erent degrees of hydrogenation, which we suggest to be the main carriers of the strong 21 km feature, and a combination of small and large partially dehydrogenated cationic PAH molecules excited by the visible radiation coming from the central star. Additional features corresponding to crystalline silicates (mostly pyroxenes) are also tentatively detected, together with a possible very broad unidentiÐed emission feature centered around 30 km, previously reported as appearing in combination with the 21 km feature in other protoÈplanetary nebulae candidates. The combination of features corresponding to O-rich and C-rich dust grains, if real, would suggest a recent change to a C-rich chemistry in the outer envelope of IRAS 16594[4656. With the help of optical ground-based spectroscopy and recently obtained Hubble Space T elescope (HST) images, we identify IRAS 16594[4656 as a new transition object between the asymptotic giant branch (post-AGB) and the planetary nebula (PN) stage. The ground-based optical spectrum shows only the Balmer lines in emission and a faint, probably shock-excited, [O I] emission over a very red continuum where strong, probably circumstellar, di †use interstellar absorption bands (DIBs) at 5780 and 6281[6284 are clearly A seen. HST WFPC2 continuum images reveal the presence of a bright central star surrounded by a bipolar reÑection nebula with a multiple-axis morphology and a maximum size of D5@@ ] 11@@.
Spectral index of the H2O-maser-emitting planetary nebula IRAS 17347-3139
Monthly Notices of The Royal Astronomical Society, 2005
We present radio continuum observations of the planetary nebula (PN) IRAS 17347-3139 (one of the only two known to harbour water maser emission), made to derive its spectral index and the turnover frequency of the emission. The spectrum of the source rises in the whole frequency range sampled, from 2.4 to 24.9 GHz, although the spectral index seems to decrease at the highest frequencies (0.79+-0.04 between 4.3 and 8.9 GHz, and 0.64+-0.06 between 16.1 and 24.9 GHz). This suggests a turnover frequency around 20 GHz (which is unusual among PNe, whose radio emission usually becomes optically thin at frequencies < 10 GHz), and a relatively high emission measure (1.5 x 10^9 cm^{-6} pc). The radio continuum emission has increased by a factor of ~1.26 at 8.4 GHz in 13 years, which can be explained as expansion of the ionized region by a factor of ~1.12 in radius with a dynamical age of ~120 yr and at an expansion velocity of ~5-40 km/s. These radio continuum characteristics, together with the presence of water maser emission and a strong optical extinction suggest that IRAS 17347-3139 is one of the youngest PNe known, with a relatively massive progenitor star.
Proceedings of the International Astronomical Union, 2011
A sample of ~150 compact Galactic PNe has been observed with the Spitzer/IRS spectrograph to characterize their dust properties. These PNe are likely to be at the onset of the PN evolutionary phase, and are therefore ideal for probing dust evolution. The molecular emission features in these Galactic PN spectra are similar to those found in our Magellanic Cloud sample, except that we found a sizable fraction of PNe with mixed-chemistry dust which are not observed in the Clouds. We also found that the distribution among dust types depends strongly on the metallicity of the parent population, implying that the metallicity of the progenitors affects the evolution of a PN from its early stages.
Time-Variable Non-Thermal Emission in the Planetary Nebula Iras 15103–5754
The Astrophysical Journal, 2015
The beginning of photoionization marks the transition between the post-Asymptotic Giant Branch (post-AGB) and planetary nebula (PN) phases of stars with masses 8 M ⊙ . This critical phase is difficult to observe, as it lasts only a few decades. The combination of jets and magnetic fields, the key agents of PNe shaping, could give rise to synchrotron emission, but this has never been observed before in any PNe, since free-free emission from the ionized gas is expected to dominate its radio spectrum. In this paper we report radio continuum observations taken with the Australia Telescope Compact Array between 1 and 46 GHz of the young PN IRAS 15103−5754. Our observations in 2010-2011 show non-thermal emission compatible with synchrotron emission from electrons accelerated at a shock with spectral index α ≃ −0.54. However, in 2012, the spectral index α ≃ −0.28 is no longer compatible with synchrotron emission in these types of processes. Several hypothesis are discussed to explain this change. The more plausible ones are related to the presence of the newly photoionized region in this young PN: either energy loss of electrons due to Coulomb collisions with the plasma, or selective suppression of synchrotron radiation due to the Razin effect. We postulate that the observed flattening of non-thermal radio spectra could be a hallmark identifying the beginning of the PN phase.
OH Maser Observations of Likely Planetary Nebulae Precursors
The Astrophysical Journal Supplement Series, 2004
We present OH maser observations at 1612, 1665, 1667, and 1720 MHz for 86 post-asymptotic giant branch (post-AGB) stars selected from a survey of 1612 MHz maser sources in the Galactic Plane. The observations were taken with the Parkes Telescope and the Australia Telescope Compact Array between 2002 September and 2003 August. Post-AGB stars are the precursors to planetary nebulae, the diverse morphological range of which is unexplained. The maser observations were taken to investigate the onset and incidence of wind asymmetries during the post-AGB phase. We re-detected all 86 sources at 1612 MHz while 27 sources were detected at 1665 and 45 at 1667 MHz. One source was re-detected at 1720 MHz. We present a classification scheme for the maser profiles and show that 25% of sources in our sample are likely to have asymmetric or bipolar outflows. From a comparison of the maser and far-infrared properties we find that there is a likely trend in the shape of the maser profiles with some sources evolving from double-peaked to irregular to fully bipolar profiles. A subset of higher-mass sources stand out as having almost no mainline emission and mostly double-peaked profiles. At least 25% of sources in the sample are variable at one or more of the frequencies observed. We also confirm a previously-noted 1667 MHz overshoot phenomenon.
IRAC Observations of Planetary Nebulae
We present the initial results from the Infrared Array Camera (IRAC) imaging survey of planetary nebulae (PN). The IRAC colors of PN are red, especially when considering the 8.0 µm band. Emission in this band is likely due to contributions from two strong H 2 lines and a [Ar III] line in that bandpass. IRAC is sensitive to the emission in the halos as well as in the ionized regions that are optically bright. In NGC 246, we have observed an unexpected ring of emission in the 5.8 and 8.0 µm IRAC bands not seen previously at other wavelengths. In NGC 650 and NGC 3132, the 8.0 µm emission is at larger distances from the central star compared to the optical and other IRAC bands, possibly related to the H 2 emission in that band and the tendency for the molecular material to exist outside of the ionized zones. In the flocculi of the outer halo of NGC 6543, however, this trend is reversed, with the 8.0 µm emission bright on the inner edges of the structures. This may be related to the emission mechanism, where the H 2 is possibly excited in shocks in the NGC 6543 halo, whereas the emission is likely fluorescently excited in the UV fields near the central star.
A Spitzer/IRS spectral study of a sample of galactic carbon-rich proto-planetary nebulae
Recent infrared spectroscopic observations have shown that proto-planetary nebulae (PPNs) are sites of active synthesis of organic compounds in the late stages of stellar evolution. This paper presents a study of Spitzer/IRS spectra for a sample of carbon-rich PPNs, all except one of which show the unidentified 21 µm emission feature. The strengths of the aromatic infrared band (AIB), 21 µm, and 30 µm features are obtained by decomposition of the spectra. The observed variations in the strengths and peak wavelengths of the features support the model that the newly synthesized organic compounds gradually change from aliphatic to aromatic characteristics as stars evolve from PPNs to planetary nebulae.
The Orion Nebula in the Mid-Infrared
The Astronomical Journal, 2005
We present two wide-field (%5 0 ; 3A5), diffraction-limited (k=D ' 0B5 at 10 m), broadband 10 and 20 m images of the Orion Nebula, plus six 7-13 m narrowband (k=Ák ' 1) images of the BN/ KL complex taken at the 3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images, centered on the Trapezium and BN/ KL regions, are mosaics of 35 00 ; 35 00 frames obtained with standard chopping and nodding techniques and reconstructed using a new restoration method developed for this project. They show the filamentary structure of the dust emission from the walls of the H ii region and reveal a new remarkable group of arclike structures %1 0 to the south of the Trapezium. The morphology of the Ney-Allen Nebula, produced by wind-wind interaction in the vicinity of the Trapezium stars, suggests a complex kinematical structure at the center of the cluster. We find indications that one of the most massive members of the cluster, the B0.5 V star 1 Ori D, is surrounded by a photoevaporated circumstellar disk. Among the four historic Trapezium OB stars, this is the only one without a binary companion, suggesting that stellar multiplicity and the presence of massive circumstellar disks may be mutually exclusive. In what concerns the BN / KL complex, we find evidence for extended optically thin silicate emission on top of the deep 10 m absorption feature. Assuming a simple two-component model, we map with '0B5 spatial resolution the foreground optical depth, color temperature, and mid-IR luminosity of the embedded sources. We resolve a conspicuous point source at the location of the IRc2-A knot, approximately 0B5 north of the deeply embedded H ii region ''I.'' We analyze the spectral profile of the 10 m silicate absorption feature and find indication for grain crystallization in the harsh nebular environment. In the OMC-1 South region, we detect several point sources and discuss their association with the mass-loss phenomenology observed at optical and millimeter wavelengths. Finally, we list the position and photometry of 177 point sources, the large majority of which are detected for the first time in the mid-IR. Twenty-two of them lack a counterpart at shorter wavelengths and are therefore candidates for deeply embedded protostars. The comparison of photometric data obtained at two different epochs reveals that source variability at 10 m is present up to a level of %1 mag on a timescale of $2 yr. With the possible exception of a pair of OB stars, all point sources detected at shorter wavelengths display 10 m emission well above the photospheric level, which we attribute to disk circumstellar emission. The recent model of Robberto et al. provides the simplest explanation for the observed mid-IR excess.
The planetary nebula nature and properties of IRAS 18197-1118
Monthly Notices of the Royal Astronomical Society, 2014
IRAS 18197−1118 is a stellar-like object that has been classified as a planetary nebula from its radio continuum emission and high [S iii]λ9532 to Paschen 9 line intensity ratio, as derived from direct images. We present intermediate-and high-resolution, optical spectroscopy, VLA 8.46 GHz radio continuum data, and narrow-band optical images of IRAS 18197−1118 aimed at confirming its planetary nebula nature, and analyzing its properties. The optical spectrum shows that IRAS 18197−1118 is a mediumexcitation planetary nebula suffering a high extinction (c Hβ ≃ 3.37). The optical images do not resolve the object but the 8.46 GHz image reveals an elliptical shell of ≃ 2.7×1.6 arcsec 2 in size, a compact central nebular region, and possible bipolar jet-like features, indicating several ejection events. The existence of a compact central nebula makes IRAS 18197-1118 singular because this kind of structure is observed in a few PNe only. An expansion velocity ≃ 20 km s −1 and a systemic velocity (LSR) ≃ +95 km s −1 are obtained for the object. An electron density of ≃ 3.4×10 4 cm −3 and an ionized mass of ≃ 2.1×10 −2 M ⊙ are deduced from the 8.46 GHz radio continuum data for an estimated statistical distance of 6 kpc. Helium abundance is high but nitrogen is not enriched, which is not consistently reproduced by evolutionary models, suggesting different abundances in the elliptical shell and central region. The properties of IRAS 18197−1118 indicate a relatively young planetary nebula, favor a distance of 6 kpc, and strongly suggest that it is an inner-disc planetary nebula.