Gordon A. H. Walker

IDENTIFICATION OF H2CCC AS A DIFFUSE INTERSTELLAR BAND CARRIER

We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450 A are caused by the B 1 B1 ← X 1 A1 transition of H2CCC (l-C3H2). The large widths of the bands are due to the short lifetime of the B 1 B1 electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450 A DIB leads to a l-C3H2 column density of ∼5 × 10 14 cm −2 toward HD 183143 and ∼2 × 10 14 cm −2 to HD 206267. Despite similar values of E(B −V), the 4881 and 5450 A DIBs in HD 204827 are less than one-third their strength in HD 183143, while the column density of interstellar C3 is unusually high for HD 204827 but undetectable for HD 183143. This can be understood if C3 has been depleted by hydrogenation to species such as l-C3H2 toward HD 183143. There are also three rotationally resolved sets of triplets of l-C3H2 in the 6150–6330 A region. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the α-band of telluric O2.

ON THE POSSIBLE ROLE OF CARBON CHAINS AS CARRIERS OF DIFFUSE INTERSTELLAR BANDS

Because the laboratory gas-phase electronic spectra of only the three polyatomic bare carbon chains C3, C4 and C5 are available, we have made a further attempt to detect the origin bands of C4 (3789 A) and C5 (5109 A) in the diffuse cloud toward ζ Oph. The measurements provide an improved 3 σ limit to their column densities: N(C4) ≤ 5 × 1011 cm-2 and N(C5) ≤ 1 × 1011 cm-2, with a signal-to-noise ratio (S/N) of 42,000 and 31,000 A-1, respectively, at a resolution of 110,000. The limits to N(C4) and N(C5) are lower than predictions from the chemical model used by Roueff and coworkers for such diffuse clouds. In conjunction with observations of related, hydrogen-containing polar chains in the millimeter region and laboratory studies of the electronic transitions of a number of homologous series, these results lead to interesting conclusions about the role of carbon chains as potential carriers of the diffuse interstellar bands (DIBs). First, the abundance and oscillator strength of smaller chains, say up to 10 atoms, are too small to account for the stronger DIBs. Second, because of the electronic configurations of these open-shell species, the lowest energy π-π transition does not have a large oscillator strength but the higher energy one in the UV does, and the chains would have to be prohibitively long for these absorptions to shift into the DIB 4000-9000 A wavelength region. The exceptions are closed-shell systems such as the odd-numbered bare carbon chains, and the ones in the C15-C31 size range that have their very strong transitions in this region. These species should be a major goal for laboratory and subsequent astronomical studies.

Limits to Interstellar C4 and C5 toward ζ Ophiuchi

We have made a sensitive search for the origin bands in the known electronic transitions of the linear carbon chains C4 and C5 at 3789 and 5109 A toward ζ Oph (Av ≤ 1). The incentive was a recent detection of C3 in this interstellar cloud with a column density of 1.6 × 1012 cm-2, plus the availability of laboratory gas phase spectra of C4 and C5. Further, some models of diffuse interstellar clouds predict that the abundance of these latter species should be within an order of magnitude of C3. Despite achieving a signal-to-noise ratio (S/N) of 2300 to 2600 per pixel at a resolution of ~110,000, the searches were negative, leading to 3 σ upper limits to the column density of N(C5)= 2 × 1011 cm-2 and N(C4)= 4 × 1012 to 1013 cm-2 where these values rely on theoretically calculated oscillator strengths. The implication of these limits is discussed along with the identification of molecules for study in future attempts to identify the carriers of the stronger diffuse interstellar bands.

Profiles of the λ6196 and λ6379 Diffuse Interstellar Bands

We have looked for rotational structure in the sharp λ6196 and λ6379 diffuse interstellar bands (DIBs) at a resolution ~120,000 in seven stars where the interstellar λ7699 K I line is unresolved. The λ6196 DIB is bell-shaped with a flat core and differs slightly in width from star to star. It is accompanied by a weak DIB at λ6194.7, with which it does not maintain a constant depth ratio. The λ6379 DIB is asymmetric with a sharp double core, but the profile hardly varies between the stars apart from being undetectable for HD 37061. Weak features connect it to the weaker λ6376 DIB, with which it varies in unison. Simple rotational models do not fit the observed profiles of λ6196 and λ6376 at all well because of more prominent branch structure in the models. We achieve an acceptable fit by arbitrarily convolving the modeled profiles with Gaussians (0.2 to 0.3 cm-1). The Gaussians correspond to an unexpected or anomalous broadening process that cannot be explained by the interstellar velocity distribution or the instrumental point-spread function. The fits give upper limits to the ratio of the rotational temperature to the moment of inertia of the molecular carriers. If the former lies in the range 10-100 K, then the molecules must be large, with moments of inertia comparable with that of the fullerene C60. We present evidence that the anomalous broadening of the rotational profiles is intramolecular in origin, but it is not easily explained by broadening processes previously invoked in connection with the DIBs. We suggest that some other process such as a zero-point vibrational isotope shift may be involved that could be characteristic of many of the narrow bands.

ASSIGNMENT OF 5069 A DIFFUSE INTERSTELLAR BAND TO HC4H+: DISAGREEMENT WITH LABORATORY ABSORPTION BAND

Krelowski et al. have reported a weak, diffuse interstellar band (DIB) at 5069 A which appears to match in both mid-wavelength and width the A 2Πu-X 2Πg gas-phase origin absorption band of HC4H+. Here, we present laboratory rotational profiles at low temperatures which are then compared with the 5069 A DIB using ~0.1 and 0.3 A line widths based on a realistic line-of-sight interstellar velocity dispersion. Neither the band shape nor the wavelength of the maximum absorption match, which makes the association of the 5069 A DIB with HC4H+ unlikely. The magnetic dipole transition X 2Πg Ω = 1/2→X 2Πg Ω = 3/2 within the ground electronic state which competes with collisional excitation is also considered. In addition, we present the laboratory gas-phase spectrum of the A 2Πu-X 2Πg transition of HC4H+ measured at 25 K in an ion trap and identify further absorption bands at shorter wavelengths for comparison with future DIB data.

Investigating Ca II Emission in the RS Canum Venaticorum Binary ER Vulpeculae Using the Broadening Function Formalism

The synchronously rotating G stars in the detached, short-period (0.7 days), partially eclipsing binary ER Vul are the most chromospherically active solar-type stars known. We have monitored activity in the Ca II H and K reversals for almost an entire orbit. Rucinskis broadening function formalism allows the photospheric contribution to be objectively subtracted from the highly blended spectra. The power of the broadening function technique is also demonstrated by the good agreement of radial velocities with those measured by others from less crowded spectral regions. In addition to strong Ca II emission from the primary and secondary, there appears to be a high-velocity stream flowing onto the secondary, where it stimulates a large active region on the surface 30°–40° in advance of the subbinary longitude. A model light curve with a spot centered on the same longitude also gives the best fit to the observed light curve. A flare with ~13% more power than at other phases was detected in one spectrum. We suggest that ER Vul may offer a magnified view of the more subtle chromospheric effects synchronized to planetary revolution seen in certain 51 Peg–type systems.

The Puzzling K and Early-M Giants: A Summary of Precise Radial Velocity Results for 15 Stars

Nearly a decade ago, “yellow giants” were introduced as a new class of low-amplitude radial-velocity variable stars. In this report we discuss new results for 12 spectral type K and early-M giants based on long-term monitoring using both the hydrogen-fluoride and iodine-cell techniques. We compare these results with those of published data for 3 additional stars ( ϒ Cephei, β Geminorum, and β Ophiuchi), and discuss possible implications for the underlying physical mechanism(s).