Diffuse Interstellar Bands Strength Research Articles

Diffuse Interstellar Bands λ6379, λ6614, and λ6660 in the LAMOST-MRS Spectra

Diffuse interstellar bands (DIBs) have always been a mysterious existence in astronomical research. In this work, we provide more DIB samples to slightly uncover this mystery. With the LAMOST medium-resolution survey (MRS) spectra, we detected three DIBs λ6379, λ6614, and λ6660, obviously superimposed on the spectra of 11,003 stars. These spectra cover spectral types from O to K, which can provide a large number of candidates for further research on DIBs. The sample shows a clear positive correlation between the strength of DIBs and extinction, which agrees with the result in the literature. In addition, there exist two peaks in each of the velocity distribution of these three DIBs, and the velocity differences between two peaks of each pair are consistent. The reason for the two velocity components is due to the Galactic rotation.

Probing the missing link between the diffuse interstellar bands and the total-to-selective extinction ratio $R_V\,\!-\!$ I. Extinction versus reddening

ABSTRACT The carriers of the still (mostly) unidentified diffuse interstellar bands (DIBs) have been a long-standing mystery ever since their first discovery exactly 100 yr ago. In recent years, the ubiquitous detection of a large number of DIBs in a wide range of Galactic and extragalactic environments has led to renewed interest in connecting the occurrence and properties of DIBs to the physical and chemical conditions of the interstellar clouds, with particular attention paid to whether the DIB strength is related to the shape of the interstellar extinction curve. To shed light on the nature and origin of the DIB carriers, we investigate the relation between the DIB strength and RV, the total-to-selective extinction ratio, which characterizes how the extinction varies with wavelength (i.e. the shape of the extinction curve). We find that the DIB strength and RV are not related if we represent the strength of a DIB by its reddening-normalized equivalent width (EW), in contrast to the earlier finding of an anticorrelation in which the DIB strength is measured by the extinction-normalized EW. This raises a fundamental question about the appropriate normalization for the DIB EW. We argue that the hydrogen column density is a more appropriate normalization than extinction and reddening.

A search of diffuse interstellar bands (DIBs) in planetary nebulae environment

Diffuse Interstellar Bands (DIBs) are set of absorption features, mostly at optical and near infrared wavelengths, that are found in the spectra of reddened stars and other objects. DIBs are actively being investigated to understand the nature of their properties and carriers and also their roles in interstellar matter enrichment. Among of the proposed carriers is large carbonaceous molecules (fullerenes). Interestingly, such as C60 and C70 have been detected in Planetary Nebulae (PN) environment. This detection allows us to study DIBs in fullerene-rich space environment. In this work, we study the presence of DIBs in central star of planetary nebulae (CSPN) by using archival data of optical spectra observed with 8.1-m Gemini South Telescope and Gemini Multi-Object Spectrographs (GMOS, spectral range of 3900 Å - 5900 Å and resolution of R ∼ 2300) and also from 2.5-m Isaac Newton Telescope with IDS spectrograph (R ∼ 2300). We measured the equivalent width of 4430, 5870 and 6283 Å DIB by fitting the detected DIB profile with the empirical template derived from higher resolution data. We investigated 4430, 5870 and 6283 Å DIBs in the spectra of CSPN and their correlation with the colour excess (E(B-V)). The correlation between the DIB strength and E(B-V) has positive value. We found that the 4430 and 5870 Å DIBs are generally more abundant on the PN environment.

Diffuse Interstellar Bands (DIBs) as a tracer of warp in the Third Galactic Quadrant: Preliminary Results

We measured 6613.6 Å diffuse interstellar band (DIB) recorded in the 318 stellar spectra of stars located in the Third Galactic Quadrant. In agreement with previous studies, we found that the DIB strength has a positive correlation with the interstellar extinction, even though significant deviations were found. The correlation study between the DIB strength and the extinction is limited by the fact that the 3D extinction survey in the Galaxy is incomplete. The DIB spatial distribution toward the South Galactic Pole indicates that the DIB carrier(s) are distributed similarly as other Galactic warp tracer. More data, especially DIB measurements from target stars above Galactic principal plane, are needed to demonstrate that DIB can be used as a tracer of Galactic warp.

Extracting diffuse interstellar bands from cool star spectra: tracing Galactic ISM structures, kinematics, and properties

Diffuse interstellar bands (DIBs) are broad and weak absorption features that appear in spectra recorded towards stars that lie behind interstellar (IS) clouds. They are known as a long-standing challenge in astronomical spectroscopy because of their unknown carriers. However, they are also a potential IS tracers, in particular to probe distant clouds because they are not easily saturated. We devised automated DIB-fitting methods appropriate for cool star spectra and multiple IS components. The data were fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. The initial number of DIB components and their radial velocity were guided by HI 21 cm emission spectrum, or, when available in the spectral range, IS neutral sodium (NaI) absorption lines. For NaI, radial velocities of IS NaI lines and DIBs were maintained linked during a global simultaneous fit. Using the tools, we have analyzed Gaia-ESO Survey (GES) spectra of stars that probe between 2-10 kpc long line-of-sight (LOS) in few different regions of the Milky Way (fields). For all fields, the DIB strength and target extinction are well correlated. We show that, toward distance-distributed target stars, DIBs can be used to locate dense clouds in the Galaxy (i.e., Galactic arm) and probe a LOS kinematical structure. In addition, we have devised the DIB-DIB global fit to study the relation between DIBs and can be a potential tool to trace interstellar environmental conditions.

Diffuse interstellar bands in the H II region M17

Context. Diffuse interstellar bands (DIBs) are broad absorption features measured in sightlines probing the diffuse interstellar medium. Although large carbon-bearing molecules have been proposed as the carriers producing DIBs, their identity remains unknown. DIBs make an important contribution to the extinction curve; the sightline to the young massive star-forming region M17 shows anomalous extinction in the sense that the total-to-selective extinction parameter (RV) differs significantly from the average Galactic value and may reach values RV > 4. Anomalous DIBs have been reported in the sightline towards Herschel 36 (RV = 5.5), in the massive star-forming region M8. Higher values of RV have been associated with a relatively higher fraction of large dust grains in the line of sight. Aims. Given the high RV values, we investigate whether the DIBs in sightlines towards young OB stars in M17 show a peculiar behaviour. Methods. We measure the properties of the most prominent DIBs in M17 and study these as a function of E(B–V) and RV. We also analyse the gaseous and dust components contributing to the interstellar extinction. Results. The DIB strengths in M17 concur with the observed relations between DIB equivalent width and reddening E(B–V) in Galactic sightlines. For several DIBs we discover a linear relation between the normalised DIB strength EW/AV and RV−1. These trends suggest two groups of DIBs: (i) a group of ten moderately strong DIBs that show a sensitivity to changes in RV that is modest and proportional to DIB strength, and (ii) a group of four very strong DIBs that react sensitively and to a similar degree to changes in RV, but in a way that does not appear to depend on DIB strength. Conclusions. DIB behaviour as a function of reddening is not peculiar in sightlines to M17. Also, we do not detect anomalous DIB profiles like those seen in Herschel 36. DIBs are stronger, per unit visual extinction, in sightlines characterised by a lower value of RV, i.e. those sightlines that contain a relatively large fraction of small dust particles. New relations between extinction normalised DIB strengths, EW/AV, and RV support the idea that DIB carriers and interstellar dust are intimately connected. Furthermore, given the distinct behaviour of two groups of DIBs, different types of carriers do not necessarily relate to the dust grains in a similar way.

Diffuse interstellar bands λ5780 and λ5797 in the Antennae Galaxy as seen by MUSE

Context. Diffuse interstellar bands (DIBs) are faint spectral absorption features of unknown origin. Research on DIBs beyond the Local Group is very limited and will surely blossom in the era of the Extremely Large Telescopes. However, we can already start paving the way. One possibility that needs to be explored is the use of high-sensitivity integral field spectrographs. Aims. Our goals are twofold. First, we aim to derive reliable mapping of at least one DIB in a galaxy outside the Local Group. Second, we want to explore the relation between DIBs and other properties of the interstellar medium (ISM) in the galaxy. Methods. We use Multi Unit Spectroscopic Explorer (MUSE) data for the Antennae Galaxy, the closest major galaxy merger. High signal-to-noise spectra were created by co-adding the signal of many spatial elements with the Voronoi binning technique. The emission of the underlying stellar population was modelled and substracted with the STARLIGHT spectral synthesis code. Flux and equivalent width of the features of interest were measured by means of fitting to Gaussian functions. Results. To our knowledge, we have derived the first maps for the DIBs at λ5780 and λ5797 in galaxies outside the Local Group. The strongest of the two DIBs (at λ5780) was detected in an area of ~0.6□′, corresponding to a linear scale of ~25 kpc2. This region was sampled using >200 out of ~1200 independent lines of sight. The DIB λ5797 was detected in >100 independent lines of sight. Both DIBs are associated with a region of high emission in the H I 21 cm line, implying a connection between atomic gas and DIBs, as the correlations in the Milky Way also suggest. Conversely, there is mild spatial association between the two DIBs and the molecular gas, in agreement with results for our Galaxy that indicate a lack of correlation between DIBs and molecular gas. The overall structures for the DIB strength distribution and extinction are comparable. Within the system, the λ5780 DIB clearly correlates with the extinction, and both DIBs follow the relationship between equivalent width and reddening when data for several galaxies are considered. This relationship is tighter when comparing only with galaxies with metallicities close to solar. Unidentified infrared emission bands (UIBs, likely caused by polycyclic aromatic hydrocarbons PAHs) and the λ5780 and λ5797 DIBs show similar but not identical spatial distributions. We attribute the differences to extinction effects without necessarily implying a radically different nature of the respective carriers. Conclusions. The results illustrate the enormous potential of integral field spectrographs for extragalactic DIB research.

Three-dimensional mapping of the local interstellar medium with composite data

Context. Three-dimensional maps of the Galactic interstellar medium are general astrophysical tools. Reddening maps may be based on the inversion of color excess measurements for individual target stars or on statistical methods using stellar surveys. Three-dimensional maps based on diffuse interstellar bands (DIBs) have also been produced. All methods benefit from the advent of massive surveys and may benefit from Gaia data. Aims. All of the various methods and databases have their own advantages and limitations. Here we present a first attempt to combine different datasets and methods to improve the local maps. Methods. We first updated our previous local dust maps based on a regularized Bayesian inversion of individual color excess data by replacing Hipparcos or photometric distances with Gaia Data Release 1 values when available. Secondly, we complemented this database with a series of ≃5000 color excess values estimated from the strength of the λ15273 DIB toward stars possessing a Gaia parallax. The DIB strengths were extracted from SDSS/APOGEE spectra. Third, we computed a low-resolution map based on a grid of Pan-STARRS reddening measurements by means of a new hierarchical technique and used this map as the prior distribution during the inversion of the two other datasets. Results. The use of Gaia parallaxes introduces significant changes in some areas and globally increases the compactness of the structures. Additional DIB-based data make it possible to assign distances to clouds located behind closer opaque structures and do not introduce contradictory information for the close structures. A more realistic prior distribution instead of a plane-parallel homogeneous distribution helps better define the structures. We validated the results through comparisons with other maps and with soft X-ray data. Conclusions. Our study demonstrates that the combination of various tracers is a potential tool for more accurate maps. An online tool makes it possible to retrieve maps and reddening estimations.

Exploring the diffuse interstellar bands with the Sloan Digital Sky Survey

We use star, galaxy and quasar spectra taken by the Sloan Digital Sky Survey to map out the distribution of diffuse interstellar bands (DIBs) induced by the Milky Way. We show that, after carefully removing the intrinsic spectral energy distribution of each source, it is possible to measure statistical flux fluctuations at the 1e-3 level, detect more than 20 DIBs and measure their strength as a function of position on the sky. We create a map of DIB absorption covering about 5000 square degrees and measure correlations with various tracers of the interstellar medium: atomic and molecular hydrogen, dust and polycyclic aromatic hydrocarbons (PAHs). After recovering known correlations, we show that each DIB has a different dependence on atomic and molecular hydrogen: while they are all positively correlated with N(HI), they exhibit a range of behaviours with N(H2) showing positive, negative or no correlation. We show that a simple parametrization involving only N(HI) and N(H2) applied to all the DIBs is sufficient to reproduce a large collection of observational results reported in the literature: it allows us to naturally describe the relations between DIB strength and dust reddening (including the so-called skin effect), the related scatter, DIB pair-wise correlations & families, the affinity for $\sigma/\zeta$-type environments and other correlations related to molecules. Our approach allows us to characterize DIB dependencies in a simple manner and provides us with a metric to characterize the similarity between different DIBs.

Using Machine Learning to classify the diffuse interstellar bands

Using over a million and a half extragalactic spectra we study the correlations of the Diffuse Interstellar Bands (DIBs) in the Milky Way. We measure the correlation between DIB strength and dust extinction for 142 DIBs using 24 stacked spectra in the reddening range E(B-V) < 0.2, many more lines than ever studied before. Most of the DIBs do not correlate with dust extinction. However, we find 10 weak and barely studied DIBs with correlations that are higher than 0.7 with dust extinction and confirm the high correlation of additional 5 strong DIBs. Furthermore, we find a pair of DIBs, 5925.9A and 5927.5A which exhibits significant negative correlation with dust extinction, indicating that their carrier may be depleted on dust. We use Machine Learning algorithms to divide the DIBs to spectroscopic families based on 250 stacked spectra. By removing the dust dependency we study how DIBs follow their local environment. We thus obtain 6 groups of weak DIBs, 4 of which are tightly associated with C2 or CN absorption lines.

Towards DIB mapping in galaxies beyond 100 Mpc

Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of unknown origin found in the spectra of stars viewed through one or several clouds of Interstellar Medium (ISM). Research of DIBs outside the Milky Way is currently very limited. Specifically spatially resolved investigations of DIBs outside of the Local Group is, to our knowledge, inexistent. Here, we explore the capability of the high sensitivity Integral Field Spectrograph, MUSE, as a tool to map diffuse interstellar bands at distances larger than 100 Mpc. We use MUSE commissioning data for AM 1353-272 B, the member with highest extinction of the "The Dentist's Chair", an interacting system of two spiral galaxies. High signal-to-noise spectra were created by co-adding the signal of many spatial elements distributed in a geometry of concentric elliptical half-rings. We derived decreasing radial profiles for the equivalent width of the $\lambda$5780.5 DIB both in the receding and approaching side of the companion galaxy up to distances of $\sim$4.6 kpc from the center of the galaxy. Likewise, interstellar extinction, as derived from the Halpha/Hbeta line ratio displays a similar trend, with decreasing values towards the external parts. This translates into an intrinsic correlation between the strength of the DIB and the extinction within AM 1353-272 B consistent with the current existing global trend between these quantities when using measurements for both Galactic and extragalactic sight lines. Mapping of DIB strength in the Local Universe as up to now only done for the Milky Way seems feasible. This offers a new approach to study the relationship between DIBs and other characteristics and species of the ISM in different conditions as those found in our Galaxy to the use of galaxies in the Local Group and/or single sightlines towards supernovae, quasars and galaxies outside the Local Group.

Dusting off the diffuse interstellar bands: DIBs and dust in extragalactic Sloan Digital Sky Survey spectra

Using over a million and a half extragalactic spectra we study the properties of the mysterious Diffuse Interstellar Bands (DIBs) in the Milky Way. These data provide us with an unprecedented sampling of the skies at high Galactic-latitude and low dust-column-density. We present our method, study the correlation of the equivalent width of 8 DIBs with dust extinction and with a few atomic species, and the distribution of four DIBs - 5780.6A, 5797.1A, 6204.3A, and 6613.6A - over nearly 15000 squared degrees. As previously found, DIBs strengths correlate with extinction and therefore inevitably with each other. However, we show that DIBs can exist even in dust free areas. Furthermore, we find that the DIBs correlation with dust varies significantly over the sky. DIB under- or over-densities, relative to the expectation from dust, are often spread over hundreds of square degrees. These patches are different for the four DIBs, showing that they are unlikely to originate from the same carrier, as previously suggested.

TheGaia-ESO Survey: Extracting diffuse interstellar bands from cool star spectra

We study how diffuse interstellar bands (DIBs) measured toward distance-distributed target stars can be used to locate dense interstellar (IS) clouds in the Galaxy and probe a line-of-sight (LOS) kinematical structure, a potential useful tool when gaseous absorption lines are saturated or not available in the spectral range. Cool target stars are numerous enough for this purpose. We have devised automated DIB fitting methods appropriate to cool star spectra and multiple IS components. The data is fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. In parallel, stellar distances and extinctions are estimated self-consistently by means of a 2D Bayesian method, from spectroscopically-derived stellar parameters and photometric data. We have analyzed Gaia-ESO Survey (GES) and previously recorded spectra that probe between $\sim$ 2 and 10 kpc long LOS in five different regions of the Milky Way. Depending on the observed spectral intervals, we extracted one or more of the following DIBs: $\lambda\lambda$ 6283.8, 6613.6 and 8620.4. For each field, we compared the DIB strengths with the Bayesian distances and extinctions, and the DIB Doppler velocities with the HI emission spectra. For all fields, the DIB strength and the target extinction are well correlated. In case of targets widely distributed in distance, marked steps in DIBs and extinction radial distance profiles match with each other and broadly correspond to the expected locations of spiral arms. For all fields, the DIB velocity structure agrees with HI emission spectra and all detected DIBs correspond to strong NaI lines. This illustrates how DIBs can be used to locate the Galactic interstellar gas and to study its kinematics at the kpc scale.

A HIGH-RESOLUTION STUDY OF NEAR-INFRARED DIFFUSE INTERSTELLAR BANDS

We present high-resolution echelle spectroscopic observations of the two near-infrared (NIR) diffuse interstellar bands (DIBs) at 13175 Å and 11797.5 Å. The DIBs have been observed in a number of diffuse interstellar medium sightlines that exhibit a wide range of visual extinctions. Band profiles are similar to those seen in narrow DIBs, clearly asymmetric and can be closely fitted in most cases using two simple Gaussian components. Gaussian fits were generally found to be more successful than fits based on a multiple-cloud model using a template DIB profile. For a sample of nine objects in which both bands are observed, the strength of both NIR DIBs generally increases with A(V), and we report a correlation between the two observed bands over a large A(V) range and widely separated lines of sight. The strength of the two bands is also compared against those of two visual DIBs and the diffuse ISM aliphatic dust absorption feature at 3.4 μm previously detected in the same sightlines. We find that the NIR DIBs do not exhibit notable (anti)correlations with either. Implications of these observations on possible DIB carrier species are discussed.

VLT/X-Shooter survey of near-infrared diffuse interstellar bands

This paper presents a spectral survey of diffuse interstellar bands (DIBs) in the NIR range, from 0.9 to 2.5 micron. The observations were designed to detect new DIBs, confirm previously proposed NIR DIBs, and characterise their behaviour with respect to known line-of-sight properties (including the optical DIBs present in our spectra). X-shooter at the VLT was used to obtained medium-resolution spectra of eight known DIB targets and one telluric reference star. In addition to the known 9577, 9632, 10780, 11797, and 13175 Angstroms NIR DIBs, we confirm 9 out of the 13 NIR DIBs that were presented by Geballe and co-workers in 2011. Furthermore, we report 12 new NIR DIB candidates. The strengths of the strongest NIR DIBs show a general correlation with reddening, E(B-V), but with a large scatter. Several NIR DIBs are more strongly correlated with the 5780 Angstroms DIB strength than with E(B-V); this is especially the case for the 15268 Angstroms DIB. The NIR DIBs are strong: the summed equivalent widths of the five strongest NIR DIBs represent a few percent of the total equivalent width of the entire average DIB spectrum (per unit reddening). The NIR DIBs towards the translucent cloud HD 147889 are all weak with respect to the general trend. No direct match was found between observed NIR DIBs and laboratory matrix-isolation spectroscopic data of PAHs. The strong correlation between the 5780-15268 DIB pair implies that (Nf)_5780 / (Nf)_15268 = 14. However, the reduced strength of the 15268 Angstroms DIB in HD 147889 rules out a common carrier for these two DIBs. Since the ionisation fraction for small PAHs in this translucent cloud is known to be low compared to diffuse clouds, the weakness of the 15268 Angstrom DIB suggests that an ionised species could be the carrier of this NIR DIB. (abridged).

Theoretical Investigation of PAHs: Implications to Diffuse Interstellar Bands

AbstractAbout a thousand optical absorption features on the interstellar extinction curve popularly known as the Diffuse Interstellar Bands (DIBs) have been observed. The numbers are increasing every year, thanks to the improvement in telescope and spectroscopic technology. Ultra-high resolution spectroscopic observations and emission features corresponding to some of the DIBs suggest that, some if not all, of these features are due to large molecules. The strength of DIBs depend on the amount of reddening which is directly proportional to the amount of material present between the background star and the observer. Since, the strengths of the DIBs are not strongly correlated with each other, there must be several carriers. Time Dependent Density Functional Theory (TDDFT) calculations are useful in narrowing down molecular systems that may be further investigated in the laboratory.The observations of the unidentified infrared (UIR) bands point towards the widespread presence of Polycyclic Aromatic Hydrocarbon (PAH) molecules. Though, not a single PAH has been discovered in interstellar space, these are the largest molecules suspected to be present. PAHs are stable towards energetic environment prevailing under interstellar conditions rendering these molecules to be good candidates as DIB carriers. We report TDDFT calculations to predict electronic transitions of neutral, protonated-deuteronated and PAHs with five member rings with various sites of protonation and deuteronation. Compared to their neutral forms, these charged isoelectronic forms of PAHs are predicted to have active transitions in the visible region, which means they are suitable candidates as carriers for some of the DIBs and laboratory studies are warranted for these systems.

SMALL-SCALE STRUCTURE OF THE INTERSTELLAR MEDIUM TOWARD ρ Oph STARS: DIFFUSE BAND OBSERVATIONS

We present an investigation of small-scale-structure in the distribution of large molecules/dust in the interstellar medium through observations of diffuse interstellar bands (DIBs). High signal-to-noise optical spectra were recorded towards the stars rho Oph A, B, C and DE using the University College London Echelle Spectrograph (UCLES) on the Anglo-Australian Telescope. The strengths of some of the DIBs are found to differ by about 5-9 % between the close binary stars rho Oph A and B, which are separated by a projected distance on the sky of only ~344 AU. This is the first star-system in which such small-scale DIB strength variations have been reported. The observed variations are attributed to differences between a combination of carrier abundance and the physical conditions present along each sightline. The sightline towards rho Oph C contains relatively dense, molecule-rich material and has the strongest \lambda\lambda 5850 and 4726 DIBs. The gas towards DE is more diffuse and is found to exhibit weak C_2 (blue) DIBs and strong yellow/red DIBs. The differences in diffuse band strengths between lines of sight are, in some cases, significantly greater in magnitude than the corresponding variations among atomic and diatomic species, indicating that the DIBs can be sensitive tracers of interstellar cloud conditions.

Detections of diffuse interstellar bands in the SDSS low-resolution spectra

Diffuse interstellar bands (DIBs) have been discovered for almost a century, but their nature remains one of the most challenging problems in astronomical spectroscopy. Most recent work to identify and investigate the properties and carriers of DIBs concentrates on high-resolution spectroscopy of selected sight-lines. In this paper, we report detections of DIBs in the Sloan Digital Sky Survey (SDSS) low-resolution spectra of a large sample of Galactic stars. Using a template subtraction method, we have successfully identified the DIBs $\lambda$$\lambda$5780, 6283 in the SDSS spectra of a sample of about 2,000 stars and measured their strengths and radial velocities. The sample is by far the largest ever assembled. The targets span a large range of reddening, E(B-V) ~ 0.2 -- 1.0, and are distributed over a large sky area and involve a wide range of stellar parameters (effective temperature, surface gravity and metallicity), confirming that the carriers of DIBs are ubiquitous in the diffuse interstellar medium (ISM). The sample is used to investigate relations between strengths of DIBs and magnitudes of line-of-sight extinction, yielding results (i.e., EW(5780)= 0.61 x E(B-V) and EW(6283) = 1.26 x E(B-V)) consistent with previous studies. DIB features have also been detected in the commissioning spectra of the Guoshoujing Telescope (LAMOST) of resolving power similar to that of SDSS. Detections of DIBs towards hundreds of thousands of stars are expected from the on-going and up-coming large scale spectroscopic surveys such as RAVE, SDSS III and LAMOST, particularly from the LAMOST Digital Sky Survey of the Galactic Anti-center (DSS-GAC). Such a huge database will provide an unprecedented opportunity to study the demographical distribution and nature of DIBs as well as using DIBs to probe the distribution and properties of the ISM and the dust extinction.

A SURVEY OF DIFFUSE INTERSTELLAR BANDS IN THE ANDROMEDA GALAXY: OPTICAL SPECTROSCOPY OF M31 OB STARS

We present the largest sample to-date of intermediate-resolution blue-to-red optical spectra of B-type supergiants in M31 and undertake the first survey of diffuse interstellar bands (DIBs) in this galaxy. Spectral classifications, radial velocities and interstellar reddenings are presented for 34 stars in three regions of M31. Radial velocities and equivalent widths are given for the 5780 and 6283 DIBs towards 11 stars. Equivalent widths are also presented for the following DIBs detected in three sightlines in M31: 4428, 5705, 5780, 5797, 6203, 6269, 6283, 6379, 6613, 6660, and 6993. All of these M31 DIB carriers reside in clouds at radial velocities matching those of interstellar Na I and/or H I. The relationships between DIB equivalent widths and reddening (E(B-V)) are consistent with those observed in the local ISM of the Milky Way. Many of the observed sightlines show DIB strengths (per unit reddening) which lie at the upper end of the range of Galactic values. DIB strengths per unit reddening are found (with 68% confidence), to correlate with the interstellar UV radiation field strength. The strongest DIBs are observed where the interstellar UV flux is lowest. The mean Spitzer 8/24 micron emission ratio in our three fields is slightly lower than that measured in the Milky Way, but we identify no correlation between this ratio and the DIB strengths in M31. Interstellar oxygen abundances derived from the spectra of three M31 H II regions in one of the fields indicate that the average metallicity of the ISM in that region is 12 + log[O/H] = 8.54 +- 0.18, which is approximately equal to the value in the solar neighbourhood.

IDENTIFICATION OF NEW NEAR-INFRARED DIFFUSE INTERSTELLAR BANDS IN THE ORION NEBULA

Large organic molecules and carbon clusters are basic building blocks of life, but their existence in the universe has not been confirmed beyond doubt. A number of unidentified absorption features (arising in the diffuse interstellar medium), usually called Diffuse Interstellar Bands (DIBs), are hypothesized to be produced by large molecules. Among these, buckminsterfullerene C60 has gained much attention as a candidate for DIB absorbers because of its high stability in space. Two DIBs at λ ~ 9577 A and 9632 A have been reported as possible features of C+ 60. However, it is still not clear how their existence depends on their environment. We obtained high-resolution spectra of three stars in/around the Orion Nebula, to search for any correlations of the DIB strength with carrier's physical conditions, such as dust abundance and UV radiation field. We find three DIBs at λ ~ 9017 A, 9210 A, and 9258 A as additional C+ 60 feature candidates, which could support this identification. These DIBs have asymmetric profiles similar to the longer wavelength features. However, we also find that the relative strengths of DIBs are close to unity and differ from laboratory measurements, a similar trend as noticed for the 9577/9632 DIBs.