EDP Sciences logo
Subscriber Authentication Point
Search
Menu
Home All issues Volume 537 (January 2012) A&A, 537 (2012) A27 Full HTML
Free Access
Issue
A&A
Volume 537, January 2012
Article Number A27
Number of page(s) 8
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201117197
Published online 22 December 2011

© ESO, 2012

1. Introduction

An infrared absorption feature at 3.4 μm is widely observed in the diffuse interstellar medium (ISM) and constitutes a major IR signature of carbonaceous interstellar dust. The feature is produced by C-H stretching vibrations in the methyl (-CH3) and methylene (-CH2-) groups of hydrocarbons. It was first detected in lines of sight toward the Galactic centre (GC) (Soifer et al. 1976; Wickramasinghe & Allen 1980; McFadzean et al. 1989) and then in other sight lines in the Milky Way (e.g. Adamson et al. 1990; Sandford et al. 1991; Pendleton et al. 1994) and in other galaxies (e.g. Bridger et al. 1994; Imanishi 2000a,b; Spoon et al. 2004; Dartois & Muñoz-Caro 2007).

A correlation has been found between the 3.4 μm optical depth τ3.4 and the visual extinction AV as expected for a carrier residing in the diffuse ISM. However, Sandford et al. (1991) noticed that this increase is not linear over the entire AV range, but rises significantly at the highest AV values (≈30−35), which are all associated with sources in the Galactic centre. The difference between Galactic centre sources and those on other sight lines was confirmed by Pendleton et al. (1994) who also observed additional sources and extended the study of τ3.4 vs. AV to lower AV. They found a ratio AV / τ3.4 ≈ 150 toward the Galactic centre and AV / τ3.4 ≈ 250 for other directions. Similar results were later obtained by Imanishi et al. (1996) and Rawlings et al. (2003). Sandford et al. (1995) concluded from this behaviour that the grains responsible for the diffuse ISM aliphatic C-H bands are different from those responsible for much of the observed visual extinction in the diffuse ISM. They also interpreted it as resulting from a non-uniform distribution of the aliphatic hydrocarbon component of the diffuse ISM throughout the Galaxy (i.e. an increase of its abundance in the inner part of the Galaxy1).

The strongest reported 3.4 μm absorption feature outside the Galactic centre region is on the line of sight to the young stellar object IRAS 18511+0146 (Ishii et al. 1998). An R ~ 1400 spectrum of this band was obtained by Ishii et al. (2002). The 3.4 μm absorption feature lies on the long-wavelength edge of the 3.1 μm water ice band absorption. The water-ice band does not arise in the diffuse ISM, but is present in the molecular cloud in which IRAS 18511+0146 is embedded.

Table 1

Log of the spectroscopic observations.

Table 2

Observed sources.

The distance to IRAS 18511+0146 has been listed as 3.9 kpc by Molinari et al. (1996) and Watt & Mundy (1999). Their values are based on the radial velocities of lines observed in the molecular cloud associated with IRAS 18511+0146. The average visual extinction associated with the foreground diffuse ISM may be estimated from the distance of IRAS 18511+0146 and the average ISM extinction vs. distance relation, 1.8 mag kpc-1 (Whittet 1992) to be AV ~ 7. If that estimation is correct, the observed optical depth of the 3.4 μm feature is three times higher than predicted by Pendleton et al. (1994) and is well outside the typical scatter in the AV:τ3.4 correlation. It therefore is of interest to further explore the diffuse ISM probed by this line of sight.

Vig et al. (2007), using multi-wavelength imaging, found that IRAS 18511+0146 is the brightest member of a cluster of stars. Here we report observations of two bright cluster members and IRAS 18511+0146, intended to determine if the apparently anomalously strong 3.4 μm band toward IRAS 18511+0146 is unique to it or is a property of the diffuse ISM in this direction. The observations and data reduction are described in Sect. 2. Results are presented in Sect. 3. The distance to the cluster is discussed in Sect. 4, and a brief discussion follows in Sect. 5.

2. Observations and data reduction

The observations were performed at the Frederick C. Gillett Gemini North telescope on Mauna Kea, using the facility near-infrared imager/spectrograph (NIRI, Hodapp et al. 2003). Spectra covering 2.95–4.15 μm were obtained on UT 2009 March 12, May 7, and May 19 (Gemini program ID: GN-2009A-Q-30). The 0.47′′ slit provided a resolving power of R ~ 690. The sky was photometric on each occasion, although the seeing differed from night to night. More information about the observations is given in Table 1.

Three sight lines in the vicinity of IRAS 18511+0146 (also known as Mol 75 and RAFGL 5542) were observed, S7, S10, and S11 (as designated by Vig et al. (2007); see their Table 2 and Fig. 5). The brightest source, S7, is IRAS 18511+0146 and is the one observed by Ishii et al. (1998) and Ishii et al. (2002). The positions and magnitudes of these sources are given in Table 2.

Spectra of each object and its calibration star (observed at similar airmass either immediately before or immediately after each cluster object) were obtained in the standard stare-nod mode, with 4′′ nods back and forth along the slit. Resultant frames were flat-fielded and corrected for bad pixels (whose locations were determined from the flat image) by interpolating over the surrounding pixels. Subtracted spectral image frames were generated from nodded pairs and combined to produce the final images. The positive and negative spectra were extracted for both the science targets and their associated standard stars and then combined to produce final raw spectra. Wavelength calibration utilised telluric absorption lines and is accurate to better than 0.001 μm. Correction was made for atmospheric and instrumental transmission by dividing the spectra of cluster objects by those of associated standard stars.

The standard stars were also used to calibrate the flux scale of the science targets. The spectra of targets divided by those of standard stars were multiplied by the spectra of blackbodies, with temperatures Teff corresponding to the individual standard stars (see Table 1), and scaled to their L-band fluxes. The L-band fluxes were derived from the K − L colours of the standard star spectral type (Tokunaga 2000); the K magnitude was obtained from the 2MASS catalog (Cutri et al. 2003). Hydrogen absorption lines in the standard stars were not removed. This results in apparent weak emission features in the final spectra at 3.04 μm (H I Pfϵ), 3.30 μm (H I Pfδ), 3.74 μm (H I Pfγ), and 4.05 μm (H I Brα) (and smaller features at 3.69 μm (H I Hu18), 3.82 μm (H I Hu16), 3.91 μm (H I Hu15), and 4.02 μm (H I Hu14).

thumbnail Fig. 1

Spectra of lines of sight toward sources S7, S10, and S11. The blue diamonds and red squares represent the data from Vig et al. (2007) (Palomar and Spitzer-IRAC data) and from Ishii et al. (1998) (MLOF and WIRO data), respectively. The dotted lines represent the local continua for the 3.1 μm water ice absorption band. Two different continua are plotted to characterize the uncertainty of the ice band depth: a blackbody function and a spline interpolation are in purple and orange (dark and light grey in the printed version), respectively.

3. Results

3.1. Overview of the spectra

The spectra of S7, S10, and S11 are presented in Figs. 1 and 2. The 3.4 μm absorption band is superimposed on the smooth long wavelength wing of the strong 3.1 μm H2O ice absorption band (see Fig. 1). That wing is represented by the dashed line in Fig. 2 and is the “local continuum” for the 3.4 μm hydrocarbon band and any additional absorption features near it (it is a polynomial of degree 5 fit to all wavelengths outside of the 3.3–3.6 μm region). For each sight line the absorption optical depth relative to the wing of the ice band has been determined by division of the spectrum (solid line) by this continuum. The optical depth spectra are shown in Fig. 3 (black solid lines) and the corresponding maximum optical depths τtot are given in Table 3. As explained in Sect. 3.2, τtot is the sum of τ3.4 from the hydrocarbon band produced in diffuse ISM and τ3.47, contributed by a broad absorption band observed in dense clouds and centred at 3.47 μm. Because the 3.47 μm band and the 3.4 μm band are partially blended on these lines of sight probing both diffuse and dense ISM, it is necessary to subtract the contribution of the 3.47 μm band in order to obtain the profile and maximum optical depth of the 3.4 μm band.

thumbnail Fig. 2

Spectra of sight lines toward sources S7, S10, and S11, between 2.95 and 4.10 μm. The dashed lines represent the local continua for the 3.4 μm absorption band.

thumbnail Fig. 3

Recovered optical depth spectra of the 3.4 μm hydrocarbon band toward sources S7, S10, and S11. The black solid lines are the optical depths deduced from the observations and the associated continua presented in Fig. 2. The deduced underlying 3.47 μm bands (dashed lines) are shown by a Gaussian functions with 0.1 μm FWHM and a maximum depth estimated from the water ice optical depths (cf. Table 3 and text for details). The red thick solid lines are the subtractions of the dashed lines from the black solid lines and represent the 3.4 μm hydrocarbon features. The positions of the subpeaks corresponding to the C-H stretching modes in the 3.4 μm band are indicated by the vertical dotted lines (asymmetric stretch in the methyl and methylene groups at 3.38 and 3.42 μm, and symmetric stretch in the methyl and methylene groups at 3.48 and 3.50 μm, respectively).

3.2. Removing the contribution of the 3.47 μm band

The 3.47 μm band is a broad (FWHM ~ 0.10   μm) and featureless absorption whose shape differs considerably from that of the 3.4 μm hydrocarbon band. Its origin and attribution are still debated (Allamandola et al. 1993; Chiar et al. 1996; Brooke et al. 1999; Dartois & d’Hendecourt 2001; Dartois et al. 2002; Mennella 2010; Bottinelli et al. 2010). The 3.47 μm optical depth τ3.47 correlates well with the depth of the 3.1 μm water ice band τ3.1. Brooke et al. (1999) found: (1)Thus, in order to correctly remove 3.47 μm bands one must estimate the optical depth of the water ice bands in our spectra. Our data do not include measurements of the continuum on the short wavelength side of the ice band; however, the data of Vig et al. (2007) provide continuum points in the K band (they and their uncertainties are shown in Fig. 1). For S7, we also used the data of Ishii et al. (1998). These data and the continua constructed from them are shown in Fig. 1. To evaluate the model-dependent uncertainties in the ice band depth, two different continua were used, as shown in the figure. Our deduced values of τ3.1 and of the corresponding uncertainties are given in Table 3. We then use the Brooke et al. (1999) relation to estimate τ3.47 (cf. Table 3) for each source. In Fig. 3, we represent the estimated underlying 3.47 μm bands (dashed lines) by Gaussian functions with 0.1 μm FWHMs. The red solid lines, which are the subtractions of the dashed lines from the black solid lines, contain the pure diffuse ISM 3.4 μm features. The corresponding optical depths are given in Table 3 by τ3.4 = τtot − τ3.47.

Table 3

Optical depths of the different absorption bands for each line of sight.

thumbnail Fig. 4

Comparison of the 3.4 μm band profile observed toward IRAS 18511+0146 (S7, thick red line) and Galactic centre sources, Sgr A* (thin black line) and GCS3 (thin blue line) (from Chiar et al. 2000).

thumbnail Fig. 5

(Left) (J − H)–(H − K) colour–colour diagram of sources within 50′′ of IRAS 18511+0146 (black dots). These sources and the associated JHKS magnitudes are found in the 2MASS All-Sky Point Source Catalogue. The red dashed lines delimit the reddening band of main sequence stars. The effect of an interstellar extinction of AV = 5 is indicated by the red arrow (Rieke & Lebofsky 1985). The loci of main sequence and giant branch dereddened stars are represented by the green crosses (Bessell & Brett 1988) and the corresponding stars reddened by AV = 5 are the red crosses. (Right) Same as the left diagram, but with sources lying within a radius of 150′′ of IRAS 18511+0146. A visual extinction vector of AV = 18 and the corresponding reddened main sequence stars are represented in blue. The positions of S7, S10, and S11 in this diagram are indicated, but for S10 and S11 only lower limits are given because of the non-detection of these source in the J band (cf. Table 2).

Note the spectral structure in the pure 3.4 μm band. The subfeatures correspond nicely to the known C-H stretching modes (indicated by the dotted vertical lines in Fig. 3): asymmetric and symmetric stretch in the methyl group (-CH3) at 3.38 and 3.48 μm, and asymmetric and symmetric stretch in the methylene group (-CH2-) at 3.42 and 3.50 μm, respectively. In Fig. 4, the 3.4 μm band profile observed toward S7 is compared to those seen in direction of the Galactic centre. This detailed agreement, not only in wavelength, but also in prominence of the substructure compared to purely diffuse sightlines, confirms that the extracted 3.4 μm band is largely due to hydrocarbons and that any under- or over-correction for the 3.47 μm band (which has no substructure) is small.

From the foregoing we conclude that the 3.4 μm band optical depth is similar in all three cluster sources and unusually strong in view of the apparent distance to the cluster.

4. The distance to IRAS 18511+0146

To properly interpret the strengths of the observed 3.4 μm absorption bands, it is critical to be certain of the distance to the IRAS 18511+0146 cluster. In this section we estimate the distance using a number of techniques.

4.1. Distances derived from Galactic rotation

Distances to objects in the Galactic disk can be estimated assuming that the observed radial velocities of the sources arise from Galactic rotation2 (e.g. Brand & Blitz 1993). For sources located in the inner Galaxy, such as IRAS 18511+0146 two values of kinematic distance from the Sun are possible. For IRAS 18511+0146, the near distance, 3.9 kpc, corresponds to the first intersection of the line of sight with the Sagittarius arm of the Galaxy. This is the distance conservatively assumed by Molinari et al. (1996). However, due to the low galactic latitude of the source, the distance from the Galactic plane does not allow the far distance to be excluded. The far distance, corresponding to the second intersection with the Sagittarius arm, is 10–10.5 kpc. Note that the far distance, when paired with the Galactic average extinction rate of 1.8 mag/kpc, corresponds to AV ~  18 mag, and the “normal” Galactic disk value of AV / τ3.4.

thumbnail Fig. 6

Optical depth of the C-H stretch band at 3.4 μm vs. visual extinction AV. The data obtained from this work are shown as red squares. The black filled circles are from McFadzean et al. (1989), Adamson et al. (1990), Sandford et al. (1991), Pendleton et al. (1994), Chiar et al. (2000), Chiar & Tielens (2001), Chiar et al. (2002), Rawlings et al. (2003), and Dartois et al. (2004). The various points associated with the Galactic centre sources have been slightly offset from each other in AV for clarity (see text for details). Left: the optical depths are τtot; i.e., they include the contribution of the 3.47 μm band. Right: the optical depths are τ3.4, due solely to the diffuse ISM (the estimated contributions of the 3.47 μm absorption have been subtracted). In both plots, the dashed line corresponds to the ratio AV / τ = 250 determined by Pendleton et al. (1994) in the local ISM. See the text for more details about the evaluation of the extinction toward S7, S10, and S11.

4.2. Distance and extinction derived from colour–colour diagrams

The interstellar extinction to IRAS 18511+0146 can be estimated using the (J − H)–(H − K) colour–colour diagrams in Fig. 5. Plotted in the left hand diagram are the sources from the 2MASS All-Sky Point Source Catalogue that are within 50′′ of IRAS 18511+0146. We adopt the infrared extinction law given by Rieke & Lebofsky (1985). Increasing reddening due only to interstellar extinction is indicated by the arrows. Main sequence stars should fall between the dashed red lines. The loci of main sequence and giant branch with no reddening are shown by the green crosses (Bessell & Brett 1988) whereas those affected by AV = 5 mag are represented in red. Sources lying to the right of the dashed lines (including the three sources discussed in this article) have local infrared excesses and their foreground extinctions are not so easily determined.

From this diagram, the positions of the main sequence stars allow us to estimate the visual extinction. By comparing the loci of observed sources that are likely main sequence stars (the black dots between red dashed lines) with the loci given by Bessell & Brett (1988) for zero extinction and with the reddening vector, we estimate AV = 5 ± 2. Given the Galactic disk extinction value of 1.8 mag/kpc, this favours the near distance for the IRAS 18511+0146 cluster.

When the colour–colour diagram is extended to include objects within 150′′ of IRAS 18511+0146 (right hand diagram in Fig. 5), sources appear in the main sequence star band reddened by considerably more than 5 mag. A reasonable interpretation is that these stars are located at the far distance discussed earlier. The clump of sources grouped around AV ≈ 5 mag is now larger. In this diagram, we have only plotted the sources detected in the J, H and KS bands, and excluded the sources where there is a lower limit on one of the three bands magnitude (except for S10 and S11). We note, however, that colour–colour diagram for the excluded sources also is consistent with two clumps of main sequence stars corresponding to the near and far distance and extinction-to-distance ratios in this direction close to the 1.8 mag/kpc average.

4.3. Other arguments

Another possible way to address the kinematic distance ambiguity is to search for gas phase atomic or molecular absorption bands toward the IRAS 18511+0146 cluster due to material in front of the cluster. Since the radial velocities of clouds along the line of sight, between the near and far distance points, are greater than the source radial velocity, the local standard of rest (LSR) velocities of absorption features can be used to distinguish between the near and far distance. The H I line at 21 cm and the H2CO line at 6 cm are often employed for this purpose. Toward IRAS 18511+0146 no absorption with velocity greater than the source vLSR is observed in H2CO (Araya et al. 2004, 2007). This tends to indicate a near distance for IRAS 18511+0146. However, the far distance cannot be excluded, because of the possible lack of formaldehyde in gas located between the near and far distances.

The detection rates of maser emission, which decrease with increasing source distance, can also provide clues for distinguishing between the near and far distance. Methanol maser emission has been detected in IRAS 18511+0146 at 6.7GHz (class II CH3OH maser) by Fontani et al. (2010) and Pandian et al. (2007) and at 44 GHz (class I CH3OH maser) by Kurtz et al. (2004). Fontani et al. (2010) have searched for class I and class II methanol masers in numerous high-mass star-forming. Their detection rates (see their Fig. 3) are  ~ 25% at d = 3–4 kpc,  ~ 10% at d = 8–10 kpc, and none for distance greater than 10 kpc. For the 44 GHz class II maser, the rate is greater than 30% at the near distance and zero at distances greater 8 kpc. These results also suggest the near distance for IRAS 18511+0146, although the possibility of unusually bright maser emission at the far distance cannot be excluded.

The luminosity of the IRAS 18511+0146 sources would be almost one order of magnitude higher if they are at the far distance rather than at the near distance. For the near distance, the luminosity of S7 is estimated at 1–2 × 104   L⊙, and between 102 and 103L⊙ for S10 and S11. The far distance would imply that S7, S10 and S11 are equivalent to O- and B-type stars. These luminosities are possible, but the close proximity of several such luminous sources is not frequent.

Finally, the radius of the IRAS 18511+0146 cluster is  ~ 0.6 − 0.7 pc for a distance of 3.9 kpc (e.g. Vig et al. 2007; Faustini et al. 2009). The far distance would imply a cluster radius of  ~ 1.6–1.8 pc. Faustini et al. (2009) have studied the properties of stellar clusters toward high-mass protostars (including IRAS 18511+0146) and have determined a median cluster radius of 0.7 pc. Only one of the 23 clusters they have detected has a radius between 1.6 and 1.8 pc; all other cluster radii are less than 1.3 pc.

We conclude that, although no individual piece of evidence is completely convincing, the collective weight of all of the evidence is strongly in favour of the near distance of 3.9 kpc for the IRAS 18511+0146 cluster. Although the far distance cannot be firmly excluded, it appears less probable, requiring simultaneously a physically large cluster containing several very luminous objects, low formaldehyde abundance along this sight line between the near and far distance points, unusually powerful methanol maser emissions both at 6.7 and 44 GHz, and the presence of main sequence stars in the near distance foreground, but no main sequence stars detected at the far distance of IRAS 18511+0146.

5. Discussion

In Fig. 6, the optical depth of the 3.4 μm band (with (left) and without (right) the 3.47 μm band contribution) is shown as a function of the diffuse interstellar extinction AV for the lines of sight observed by McFadzean et al. (1989), Adamson et al. (1990), Sandford et al. (1991), Pendleton et al. (1994), Chiar et al. (2000), Chiar & Tielens (2001), Chiar et al. (2002), Rawlings et al. (2003), and Dartois et al. (2004). Our estimates for S7, S10, and S11 are shown by red squares; there locations in the plots assume an interstellar visual extinction AV ≈ 7 estimated using the most probable distance, 3.9 kpc, and the standard extinction to distance ratio, as discussed earlier. Note in the figure that the values of τ3.4 corresponding to AV = 19 and AV = 22 are particularly uncertain. This is because the spectra from which those values of τ3.4 were determined contain strong photospheric lines of OH near 3.4 μm that blend with the hydrocarbon feature and also because circumstellar dust may also contribute to the extinction (Sandford et al. 1991, 1995).

The interstellar visual extinction toward the Galactic centre is usually assumed to be around 30 mag but is known to vary on small spatial scales over the range 25 ≲ AV ≲ 40 (e.g. Cotera et al. 2000; Rieke et al. 1989, and references therein). Because of the high extinction, the Galactic centre is unobservable at visible wavelengths and AV cannot be directly measured. The extinction must thus be determined in the near-infrared and then extrapolated to optical wavelengths using an extinction law to obtain AV. The extinction to the Galactic centre in the KS band has been found to be around 3 mag (e.g. Cotera et al. 2000). Schödel et al. (2010) recently determined a mean extinction toward the central parsec of the Galaxy of AKs = 2.54. The most commonly used conversion factor from near-infrared to visual is the ratio AV / AKs ~ 9 found by Rieke & Lebofsky (1985), but more recent studies toward the GC indicate higher values (AV / AKs ~ 16, Nishiyama et al. 2008). As remarked by Gosling et al. (2009), this steeper extinction law slope suggests that previous conversions of near-infrared extinction to AV could have underestimated the values of AV. Note also that the Galactic centre sightline includes spiral arms and therefore possible dense cloud components that could contribute 10 visual magnitudes (Whittet et al. 1997). Thus, the exact value of AV arising in the diffuse ISM toward the Galactic centre is difficult to determine, and the relation between AV and the column density of matter may not be valid as a proxy common to both diffuse ISM and Galactic centre directions. The corresponding data points in Fig. 6 are plotted with AV ~ 30 but their exact positions remain quite uncertain.

In the right hand diagram of Fig. 6, the contribution of the dense ISM τ3.47 has been estimated from τice and subtracted in order to obtain only the diffuse ISM contribution to the absorption. The subtraction has been done not only for the IRAS 18511+0146 sources, but also for those in direction of the Galactic centre, where water ice absorption is also observed. Note that the values of τ3.4 obtained Galactic centre sources are still above the local ISM correlation (the dashed line, i.e. AV / τ = 250 determined by Pendleton et al. 1994), but the gap between the Galactic centre and local ISM optical depths is significantly smaller when subtracting the 3.47 μm band contribution.

It is clear from Fig. 6 that the optical depths of the 3.4 μm hydrocarbon bands toward the IRAS 18511+0146 cluster sources do not follow the linear correlation with the diffuse interstellar extinction observed in the Galactic disk, if the corresponding extinction is indeed AV ≈ 7, but exceed the predicted value by approximately a factor of three. Our understanding of this depends on whether or not the extinction to the young stellar objects in the cluster is the same as that inferred both by the colour-colour diagrams in Fig. 5 for the main sequence stars near the cluster and by the distance combined with the average Galactic AV to distance value. As underlined by Ishii et al. (2002), interstellar extinction is highly patchy and the AV to distance ratio can be higher in some particular lines of sight. Moreover, one might expect that remnant diffuse gas would be associated with young clusters and that both their visual extinctions and values of τ3.4 could be higher than predicted from their distances and the average values for Galactic disk extinction vs. distance. For example, toward the young and massive star Cyg OB2 12, which is physically associated with a distinct cloud of diffuse gas, the extinction ratio is 6 ± 1 mag kpc-1 (Whittet et al. 1997, and references therein). Values of AV and τ3.4 would be expected to be more normal for field stars.

For diffuse interstellar gas on the sightline to the IRAS 18511+0146 cluster, an average extinction of 4–5 mag kpc-1, corresponding to a visual extinction of 15–20 mag, would produce the strong 3.4 μm hydrocarbon bands that are observed towards S7, S10, and S11. Further studies of the gas associated with the young cluster are needed to determine if much of the extinction in the diffuse ISM is local to the cluster. Alternatively, observations of stars in front of the IRAS 18511+0146 cluster could determine whether the excess optical extinction and possibly the excess 3.4 μm band strength arise in other locations along the line of sight.

6. Conclusion

We have observed the 3.4 μm absorption band, due to aliphatic C-H stretching modes of interstellar hydrogenated amorphous carbon dust, in three closely spaced lines of sight to young stars in the IRAS 18511+0146 cluster. These observations have confirmed the high optical depth of that band in the diffuse ISM in this direction and have revealed little or no source-to-source variation in it. The 3.4 μm optical depths observed toward these sources are the highest observed in directions other than toward the Galactic centre.

We present several independent observational constraints favouring the previously adopted distance to the cluster of 3.9 kpc, corresponding to the near intersection of the line of sight with the Sagittarius Arm, as opposed to the location of the cluster at far intersection, at a distance of 10–10.5 pc. If the IRAS 18511+0146 cluster is at the far distance, which can still not be firmly excluded but is less probable, this direction would not probe an unusual ISM, but still provide privileged lines of sight to observe high quantities of diffuse ISM dust. Assuming the more probable near distance, the large depth of the 3.4 μm band may be interpreted in two simple ways:

  • (i)

    The cluster is associated with a local diffuse cloud that providesexcess 3.4 μm optical depth and visual extinction AV above the amounts predicted by the standard ratio of Galactic distance to AV, and with the standard Galactic ratio AV / τ3.4 ≈ 250 satisfied.

  • (ii)

    There is no local diffuse cloud associated with the cluster, and the diffuse ISM probed by this line of sight contains an unusually high abundance of the carrier of the 3.4 μm feature, as shown in Fig. 6.

Further estimates of the distance and the extinction to the IRAS 18511+0146 cluster are needed to properly interpret the high 3.4 μm optical depths. In either case, the high τ3.4 makes this cluster a unique and valuable line of sight for studying Galactic hydrocarbon dust.

1

This could also be interpreted as a decrease of the non-aliphatic extinction-producing material in the central regions of the Galaxy.

2

Observed radial velocities are usually assumed to have deviations from pure circular rotation by up to 10 km s.

Acknowledgments

This paper is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). This work was in part supported by the ANR COSMISME project, grant ANR-2010-BLAN-0502 of the French Agence Nationale de la Recherche. G.M.M.C. was supported by project AYA2008-06374 funded by Spanish MICINN.

References

  1. Adamson, A. J., Whittet, D. C. B., & Duley, W. W. 1990, MNRAS, 243, 400 [NASA ADS] [Google Scholar]
  2. Allamandola, L. J., Sandford, S. A., Tielens, A. G. G. M., & Herbst, T. M. 1993, Science, 260, 64 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  3. Araya, E., Hofner, P., Linz, H., et al. 2004, ApJS, 154, 579 [NASA ADS] [CrossRef] [Google Scholar]
  4. Araya, E., Hofner, P., Goss, W. M., et al. 2007, ApJS, 170, 152 [NASA ADS] [CrossRef] [Google Scholar]
  5. Balachandran, S. 1990, ApJ, 354, 310 [NASA ADS] [CrossRef] [Google Scholar]
  6. Bessell, M. S., & Brett, J. M. 1988, PASP, 100, 1134 [NASA ADS] [CrossRef] [Google Scholar]
  7. Boesgaard, A. M., & Tripicco, M. J. 1986, ApJ, 303, 724 [NASA ADS] [CrossRef] [Google Scholar]
  8. Bottinelli, S., Adwin Boogert, A. C., Bouwman, J., et al. 2010, ApJ, 718, 1100 [NASA ADS] [CrossRef] [Google Scholar]
  9. Brand, J., & Blitz, L. 1993, A&A, 275, 67 [NASA ADS] [Google Scholar]
  10. Bridger, A., Wright, G. S., & Geballe, T. R. 1994, in Infrared Astronomy with Arrays: The Next Generation, ed. I. S. McLean, Astrophysics and Space Science Library, 190, 537 [Google Scholar]
  11. Brooke, T. Y., Sellgren, K., & Geballe, T. R. 1999, ApJ, 517, 883 [NASA ADS] [CrossRef] [Google Scholar]
  12. Chiar, J. E., & Tielens, A. G. G. M. 2001, ApJ, 550, L207 [NASA ADS] [CrossRef] [Google Scholar]
  13. Chiar, J. E., Adamson, A. J., & Whittet, D. C. B. 1996, ApJ, 472, 665 [NASA ADS] [CrossRef] [Google Scholar]
  14. Chiar, J. E., Tielens, A. G. G. M., Whittet, D. C. B., et al. 2000, ApJ, 537, 749 [NASA ADS] [CrossRef] [Google Scholar]
  15. Chiar, J. E., Adamson, A. J., Pendleton, Y. J., et al. 2002, ApJ, 570, 198 [NASA ADS] [CrossRef] [Google Scholar]
  16. Cotera, A. S., Simpson, J. P., Erickson, E. F., et al. 2000, ApJS, 129, 123 [NASA ADS] [CrossRef] [Google Scholar]
  17. Cutri, R. M., Skrutskie, M. F., van Dyk, S., et al. 2003, 2MASS All Sky Catalog of point sources., ed. R. M. Cutri, M. F. Skrutskie, S. van Dyk, et al. (NASA/IPAC Infrared Science Archive) [Google Scholar]
  18. Dartois, E., & d’Hendecourt, L. 2001, A&A, 365, 144 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  19. Dartois, E., & Muñoz-Caro, G. M. 2007, A&A, 476, 1235 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  20. Dartois, E., d’Hendecourt, L., Thi, W., Pontoppidan, K. M., & van Dishoeck, E. F. 2002, A&A, 394, 1057 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  21. Dartois, E., Marco, O., Muñoz-Caro, G. M., et al. 2004, A&A, 423, 549 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  22. Faustini, F., Molinari, S., Testi, L., & Brand, J. 2009, A&A, 503, 801 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  23. Fontani, F., Cesaroni, R., & Furuya, R. S. 2010, A&A, 517, A56 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  24. Gosling, A. J., Bandyopadhyay, R. M., & Blundell, K. M. 2009, MNRAS, 394, 2247 [NASA ADS] [CrossRef] [Google Scholar]
  25. Hodapp, K. W., Jensen, J. B., Irwin, E. M., et al. 2003, PASP, 115, 1388 [NASA ADS] [CrossRef] [Google Scholar]
  26. Imanishi, M. 2000a, MNRAS, 313, 165 [NASA ADS] [CrossRef] [Google Scholar]
  27. Imanishi, M. 2000b, MNRAS, 319, 331 [NASA ADS] [CrossRef] [Google Scholar]
  28. Imanishi, M., Sasaki, Y., Goto, M., et al. 1996, AJ, 112, 235 [NASA ADS] [CrossRef] [Google Scholar]
  29. Ishii, M., Nagata, T., Sato, S., et al. 1998, AJ, 116, 868 [NASA ADS] [CrossRef] [Google Scholar]
  30. Ishii, M., Nagata, T., Chrysostomou, A., & Hough, J. H. 2002, AJ, 124, 2790 [NASA ADS] [CrossRef] [Google Scholar]
  31. Kurtz, S., Hofner, P., & Álvarez, C. V. 2004, ApJS, 155, 149 [NASA ADS] [CrossRef] [Google Scholar]
  32. McFadzean, A. D., Whittet, D. C. B., Bode, M. F., Adamson, A. J., & Longmore, A. J. 1989, MNRAS, 241, 873 [Google Scholar]
  33. Mennella, V. 2010, ApJ, 718, 867 [NASA ADS] [CrossRef] [Google Scholar]
  34. Molinari, S., Brand, J., Cesaroni, R., & Palla, F. 1996, A&A, 308, 573 [NASA ADS] [Google Scholar]
  35. Nishiyama, S., Nagata, T., Tamura, M., et al. 2008, ApJ, 680, 1174 [NASA ADS] [CrossRef] [Google Scholar]
  36. Pandian, J. D., Goldsmith, P. F., & Deshpande, A. A. 2007, ApJ, 656, 255 [NASA ADS] [CrossRef] [Google Scholar]
  37. Pendleton, Y. J., Sandford, S. A., Allamandola, L. J., Tielens, A. G. G. M., & Sellgren, K. 1994, ApJ, 437, 683 [NASA ADS] [CrossRef] [Google Scholar]
  38. Rawlings, M. G., Adamson, A. J., & Whittet, D. C. B. 2003, MNRAS, 341, 1121 [NASA ADS] [CrossRef] [Google Scholar]
  39. Rieke, G. H., & Lebofsky, M. J. 1985, ApJ, 288, 618 [NASA ADS] [CrossRef] [Google Scholar]
  40. Rieke, G. H., Rieke, M. J., & Paul, A. E. 1989, ApJ, 336, 752 [NASA ADS] [CrossRef] [Google Scholar]
  41. Sandford, S. A., Allamandola, L. J., Tielens, A. G. G. M., et al. 1991, ApJ, 371, 607 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  42. Sandford, S. A., Pendleton, Y. J., & Allamandola, L. J. 1995, ApJ, 440, 697 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  43. Schödel, R., Najarro, F., Muzic, K., & Eckart, A. 2010, A&A, 511, A18 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  44. Soifer, B. T., Russell, R. W., & Merrill, K. M. 1976, ApJ, 207, L83 [NASA ADS] [CrossRef] [Google Scholar]
  45. Spoon, H. W. W., Armus, L., Cami, J., et al. 2004, ApJS, 154, 184 [NASA ADS] [CrossRef] [Google Scholar]
  46. Tokunaga, A. T. 2000, Infrared Astronomy, 4th ed., ed. Cox, A. N. (New York: AIP Press; Springer), 143 [Google Scholar]
  47. Vig, S., Testi, L., Walmsley, M., et al. 2007, A&A, 470, 977 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  48. Watt, S., & Mundy, L. G. 1999, ApJS, 125, 143 [NASA ADS] [CrossRef] [Google Scholar]
  49. Whittet, D. C. B. 1992, Dust in the galactic environment (Institute of Physics Publishing) [Google Scholar]
  50. Whittet, D. C. B., Boogert, A. C. A., Gerakines, P. A., et al. 1997, ApJ, 490, 729 [NASA ADS] [CrossRef] [Google Scholar]
  51. Wickramasinghe, D. T., & Allen, D. A. 1980, Nature, 287, 518 [NASA ADS] [CrossRef] [Google Scholar]

All Tables

Table 1

Log of the spectroscopic observations.

In the text
Table 2

Observed sources.

In the text
Table 3

Optical depths of the different absorption bands for each line of sight.

In the text

All Figures

thumbnail Fig. 1

Spectra of lines of sight toward sources S7, S10, and S11. The blue diamonds and red squares represent the data from Vig et al. (2007) (Palomar and Spitzer-IRAC data) and from Ishii et al. (1998) (MLOF and WIRO data), respectively. The dotted lines represent the local continua for the 3.1 μm water ice absorption band. Two different continua are plotted to characterize the uncertainty of the ice band depth: a blackbody function and a spline interpolation are in purple and orange (dark and light grey in the printed version), respectively.
In the text
thumbnail Fig. 2

Spectra of sight lines toward sources S7, S10, and S11, between 2.95 and 4.10 μm. The dashed lines represent the local continua for the 3.4 μm absorption band.
In the text
thumbnail Fig. 3

Recovered optical depth spectra of the 3.4 μm hydrocarbon band toward sources S7, S10, and S11. The black solid lines are the optical depths deduced from the observations and the associated continua presented in Fig. 2. The deduced underlying 3.47 μm bands (dashed lines) are shown by a Gaussian functions with 0.1 μm FWHM and a maximum depth estimated from the water ice optical depths (cf. Table 3 and text for details). The red thick solid lines are the subtractions of the dashed lines from the black solid lines and represent the 3.4 μm hydrocarbon features. The positions of the subpeaks corresponding to the C-H stretching modes in the 3.4 μm band are indicated by the vertical dotted lines (asymmetric stretch in the methyl and methylene groups at 3.38 and 3.42 μm, and symmetric stretch in the methyl and methylene groups at 3.48 and 3.50 μm, respectively).
In the text
thumbnail Fig. 4

Comparison of the 3.4 μm band profile observed toward IRAS 18511+0146 (S7, thick red line) and Galactic centre sources, Sgr A* (thin black line) and GCS3 (thin blue line) (from Chiar et al. 2000).
In the text
thumbnail Fig. 5

(Left) (J − H)–(H − K) colour–colour diagram of sources within 50′′ of IRAS 18511+0146 (black dots). These sources and the associated JHKS magnitudes are found in the 2MASS All-Sky Point Source Catalogue. The red dashed lines delimit the reddening band of main sequence stars. The effect of an interstellar extinction of AV = 5 is indicated by the red arrow (Rieke & Lebofsky 1985). The loci of main sequence and giant branch dereddened stars are represented by the green crosses (Bessell & Brett 1988) and the corresponding stars reddened by AV = 5 are the red crosses. (Right) Same as the left diagram, but with sources lying within a radius of 150′′ of IRAS 18511+0146. A visual extinction vector of AV = 18 and the corresponding reddened main sequence stars are represented in blue. The positions of S7, S10, and S11 in this diagram are indicated, but for S10 and S11 only lower limits are given because of the non-detection of these source in the J band (cf. Table 2).
In the text
thumbnail Fig. 6

Optical depth of the C-H stretch band at 3.4 μm vs. visual extinction AV. The data obtained from this work are shown as red squares. The black filled circles are from McFadzean et al. (1989), Adamson et al. (1990), Sandford et al. (1991), Pendleton et al. (1994), Chiar et al. (2000), Chiar & Tielens (2001), Chiar et al. (2002), Rawlings et al. (2003), and Dartois et al. (2004). The various points associated with the Galactic centre sources have been slightly offset from each other in AV for clarity (see text for details). Left: the optical depths are τtot; i.e., they include the contribution of the 3.47 μm band. Right: the optical depths are τ3.4, due solely to the diffuse ISM (the estimated contributions of the 3.47 μm absorption have been subtracted). In both plots, the dashed line corresponds to the ratio AV / τ = 250 determined by Pendleton et al. (1994) in the local ISM. See the text for more details about the evaluation of the extinction toward S7, S10, and S11.
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.