44 7 v 1 2 4 A ug 2 00 4 Reconsidering the origin of the 21 micron feature : Oxides in carbon - rich PPNe ?
نویسندگان
چکیده
The origin of the so-called '21' µm feature which is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNe) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. The present paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides. SiC is shown to produce a strong resonance band at 20–21 µm if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO 2 indeed have their strongest spectral signature at a position of 20.1 µm, which coincides with the position of the '21' µm emission band. The optical constants of another candidate carrier that has been relatively neglected so far – iron monoxide – are proven to permit a fairly accurate reproduction of the '21' µm feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the '21' µm emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that the feature, too, is detected in a small group of objects only. However, it is unclear how FeO should form or survive particularly in carbon-rich PPNe.
منابع مشابه
ar X iv : a st ro - p h / 02 07 44 7 v 3 6 M ay 2 00 3 Application of MHD Disk Wind Solutions to Planetary / Protoplanetary Nebulae
Winds from accretion disks have been proposed as the driving source for precessing jets and extreme bipolar morphologies in Planetary Nebulae (PNe) and proto-PNe (pPNe). In this paper we address the applicability MHD disk wind models to PNe and pPNe. We first review Pelletier & Pudritz's (1992) non self-similar approach to magneto-centrifugal disk winds. We then discuss conditions which may occ...
متن کاملar X iv : 0 80 3 . 44 39 v 2 [ m at h . D S ] 4 A ug 2 00 8 PERIODIC UNIQUE BETA - EXPANSIONS : THE SHARKOVSKIĬ ORDERING
Let β ∈ (1, 2). Each x ∈ [0, 1 β−1 ] can be represented in the form
متن کاملLine 803 (12/11/16) -- Metro Rail - Metro Green Line - RailEastbound to Norwalk - Westbound to Redondo Beach
— — — — — — — — — — 3:36A 3:42A 3:58A 4:08A — 3:53A 3:55A 3:57A 3:59A 4:02A 4:05A 4:07A 4:09A 4:11A 4:13 4:16 4:21 4:25 — 4:21 4:23 4:25 4:27 4:30 4:33 4:35 4:37 4:39 4:41 4:44 4:49 4:53 4:31A 4:33 4:35 4:37 4:39 4:42 4:45 4:47 4:49 4:51 5:00 5:03 5:08 5:12 4:50 4:52 4:54 4:56 4:58 5:01 5:04 5:06 5:08 5:10 5:12 5:15 5:20 5:24 5:03 5:05 5:07 5:09 5:11 5:14 5:17 5:19 5:21 5:23 5:25 5:28 5:33 5:37...
متن کاملOn the Carriers of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21μm emission feature seen in sixteen C-rich proto-planetary nebulae (PPNe) remains unidentified since its discovery in 1989. Over a dozen of materials are suggested as the carrier candidates. In this work we quantitatively investigate eight inorganic and one organic carrier candidates in terms of elemental abundance constraints, while previous studies mostly focus on their spect...
متن کامل