Context. The identification of the carriers of the diffuse interstellar bands (DIBs) is the longest standing problem in the study of the interstellar medium. Here we present recent UV laboratory ...spectra of various polycyclic aromatic hydrocarbons (PAHs) and explore the potential of these molecules as carriers of the DIBs. Whereas, in the near IR range, the PAHs exhibit vibrational bands that are not molecule-specific, their electronic transitions occurring in the UV/vis provide characteristic fingerprints. The comparison of laboratory spectra calibrated in intensity with high signal-to-noise observational data in the UV enables us to establish new constraints on PAH abundances. Aims. From a detailed comparison of the gas-phase and Ne-matrix absorption spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, benzoghiperylene, and hexabenzocoronene with new interstellar spectra, we aim to infer the abundance of these PAHs in the interstellar medium. Methods. We present spectra of PAHs measured at low temperature in the gas phase and in an Ne matrix, and present methods to derive absolute absorption cross sections for the matrix and gas phase spectra. We have obtained high signal to noise (S/N > 100) absorption spectra toward five lines of sight with reddenings of EB−V = 1−1.6 mag. The spectra cover the 3050−3850 Å wavelength region where the PAHs studied here show prominent absorption features. Results. From the observations, we infer upper limits in the fractional abundances of the PAHs studied here. Upper limits in the column densities of anthracene of 0.8−2.8 × 1012 cm-2 and of pyrene and 2,3-benzofluorene ranging from 2−8 × 1012 cm-2 are inferred. Upper limits in the column densities of benzoghiperylene are 0.9−2.4 × 1013 and 1014 cm-2 for phenanthrene. The measurements indicate fractional abundances of anthracene, pyrene, and 2,3-benzofluorene of a few times 10-10. Upper limits in the fractional abundance of benzoghiperylene of a few times 10-9 and of phenanthrene of few times 10-8 are inferred. Toward CPD −32°1734, we found near 3584 Å an absorption line of OH+, which was discovered in the interstellar medium only very recently. Conclusions. The fractional abundances of PAHs inferred here are up to two orders of magnitude lower than estimated total PAH abundances in the interstellar medium. This indicates that either neutral PAHs are not abundant in translucent molecular clouds or that a PAH population with a wide variety of molecules is present.
Context.Molecular hydrogen observations towards Herbig-Haro objects provide the possibility of studying physical processes related to star formation. Aims.Observations towards the luminous IRAS ...source IRAS 11101-5928 and the associated Herbig-Haro objects HH135/HH136 are obtained to understand whether high-mass stars form via the same physical processes as their low-mass counterparts. Methods.Near-infrared imaging and spectroscopy are used to infer H2 excitation characteristics. A theoretical H2 spectrum is constructed from a thermal ro-vibrational population distribution and compared to the observations. Results.The observations reveal the presence of a well-collimated, parsec-sized H2 outflow with a total H2 luminosity of about $2~L_\odot$. The bulk of the molecular gas is characterized by a ro-vibrational excitation temperature of $2000\pm200$ K. A small fraction (0.3%) of the molecular gas is very hot, with excitation temperatures around 5500 K. The molecular emission is associated with strong FeII emission. The H2 and FeII emission characteristics indicate the presence of fast, dissociative J-shocks at speeds of $v_\mathrm{s} \approx 100$ km s-1. Electron densities of $n_\mathrm{e} = 3500$–4000 cm-3 are inferred from the FeII line ratios. Conclusions.The large H2 luminosity combined with the very large source luminosity suggests that the high-mass protostar that powers the HH135/HH136 flow forms via accretion, but with a significantly increased accretion rate compared to that of low-mass protostars.
We discuss the abundances of interstellar CH, CH+, and CN in the Magellanic Clouds, derived from spectra of seven SMC and 13 LMC stars obtained (mostly) with the VLT UVES. CH and/or CH+ have now been ...detected toward three SMC and nine LMC stars; CN is detected toward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly all the optical detections of these molecular species in interstellar media beyond the Milky Way. In the LMC, the CH/H2 ratio is comparable to that found for diffuse Galactic molecular clouds in four sight lines but is lower by factors of 2.5-4.0 in two others. In the SMC, the CH/H2 ratio is comparable to the local Galactic value in one sight line but is lower by factors of 10-15 in two others. The abundance of CH in the Magellanic Clouds thus appears to depend on local physical conditions and not just on metallicity. In both the SMC and the LMC, the observed relationships between the column density of CH and those of CN, CH+, Na I, and K I are generally consistent with the trends observed in our Galaxy. Using existing data for the rotational populations of H2 in these sight lines, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. The inferred temperatures range from about 45 to 90 K, the radiation fields range from about 1 to 900 times the typical local Galactic field, and the densities (in most cases) lie between 100 and 600 cm-3. Densities estimated from the observed N(CH), under the assumption that CH is produced via steady state gas-phase reactions, are considerably higher than those derived from H2. Much better agreement is found by assuming that the CH is made via the (still undetermined) process(es) responsible for the observed CH+. A significant fraction of the CH and CH+ in diffuse molecular material in the SMC and LMC may be produced in photon-dominated regions. The excitation temperature obtained from the populations of the two lowest CN rotational levels toward Sk -67 2 is quite consistent with the temperature of the cosmic microwave background radiation measured with COBE. Toward most of our targets, the UVES spectra also reveal absorption at velocities corresponding to the Magellanic Clouds ISM from several of the strongest of the diffuse interstellar bands (DIBs; at 5780, 5797, and 6284 AA). On average, the three DIBs are weaker by factors of 7-9 (LMC) and about 20 (SMC), compared to those typically observed in Galactic sight lines with similar N(H I), presumably due to the lower metallicities and stronger radiation fields in the LMC and SMC. The three DIBs are also weaker (on average, but with some exceptions), by factors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in the Magellanic Clouds. The detection of several of the so-called C2 DIBs toward Sk 143 and Sk -67 2 with strengths similar to those in comparable Galactic sight lines, however, indicates that no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or for all sight lines) in the Magellanic Clouds.
Context. High redshift radio galaxies are among the most massive galaxies at their redshift, are often found at the center of protoclusters of galaxies, and are expected to evolve into the present ...day massive central cluster galaxies. Thus they are a useful tool to explore structure formation in the young Universe. Aims. 3C 294 is a powerful FR II type radio galaxy at z = 1.786. Past studies have identified a clumpy structure, possibly indicative of a merging system, as well as tentative evidence that 3C 294 hosts a dual active galactic nucleus (AGN). Due to its proximity to a bright star, it has been subject to various adaptive optics imaging studies. Methods. In order to distinguish between the various scenarios for 3C 294, we performed deep, high-resolution adaptive optics near-infrared imaging and optical spectroscopy of 3C 294 with the Large Binocular Telescope. Results. We resolve the 3C 294 system into three distinct components separated by a few tenths of an arcsecond on our images. One is compact, the other two are extended, and all appear to be non-stellar. The nature of each component is unclear. The two extended components could be a galaxy with an internal absorption feature, a galaxy merger, or two galaxies at different redshifts. We can now uniquely associate the radio source of 3C 294 with one of the extended components. Based on our spectroscopy, we determined a redshift of z = 1.784 ± 0.001, which is similar to the one previously cited. In addition we found a previously unreported emission line at λ6749.4 Å in our spectra. It is not clear that it originates from 3C 294. It could be the Ne IV doublet λ2424/2426 Å at z = 1.783, or belong to the compact component at a redshift of z ∼ 4.56. We thus cannot unambiguously determine whether 3C 294 hosts a dual AGN or a projected pair of AGNs.
An analysis of optical echelle spectra towards nine stars in the Cep OB4 association is presented. Interstellar absorption lines which arise in the (1, 0) and (2, 0) bands of the CN A $^2\Pi$ – ...X $^2\Sigma^+$ red system towards BD+66° 1661, BD+66° 1674, and BD+66° 1675 are used to infer accurate CN column densities N(CN). A comparison with earlier measurements in the CN violet system allows to infer a CN Doppler b parameter of b(CN$) = 1.2{-}2.4$ km s-1. Molecular carbon absorption lines which arise in the (1, 0), (2, 0) and (3, 0) bands of the C2 A$^1\Pi_{\rm u}$ – X$^1\Sigma_{\rm g}^+$ Phillips system is used to infer gaskinetic temperatures of $35\pm10$ K and densities of $n = 700 \pm 200$ cm-3 towards BD+66° 1661 and BD+66° 1675, and a temperature of $60\pm10$ K and a density of $800 \pm 400$ cm-3 towards BD+66° 1674. The R(1) line of the (0, 0) band of the CH A$^2\Delta - {\rm X}^2\Pi$ system is detected towards 6 stars. A tight correlation exists between N(C2) and N(CH). N(CN) increases with N(CH) and with N(C2). Interstellar CH+ is marginally detected towards four stars. Inferred CH+ column densities are significantly lower than towards other lines of sight with similar reddening. The velocity structure towards Cep OB4 and the chemical abundances suggest that CN, C2, CH, and CH+ are formed in quiescent material. A previous suggestion that the molecules form in a photon-dominated region close to the stars is not supported by the observations.
Context. Accretion outbursts are key elements in star formation. ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest-mass star known to experience accretion outbursts. Since its ...discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. Aims. We explore the photometric and spectroscopic behavior of ASASSN-13db during the 2014–2017 outburst. Methods. We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the light curve of ASASSN-13db and the dynamical and physical properties of the accretion flow. Results. The 2014–2017 outburst lasted for nearly 800 days. A 4.15 d period in the light curve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blue-shifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of ~5800–6000 K in the accretion flow. Conclusions. Photometrically and spectroscopically, the 2014–2017 event displays an intermediate behavior between EXors and FUors. The accretion rate (Ṁ= 1–3 × 10-7 M⊙/yr), about two orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very-low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settle after the beginning of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power of time-resolved photometry and spectroscopy to explore the properties of variable and outbursting stars.
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