A search for naphthalene in diffuse interstellar clouds
Susana Iglesias-Groth, Jonay I. González Hernández, Arturo Manchado
aa r X i v : . [ a s t r o - ph . GA ] S e p Mon. Not. R. Astron. Soc. , 1– ?? (2010) Printed 20 November 2018 (MN L A TEX style file v2.2)
A search for naphthalene in diffuse interstellar clouds
Susana Iglesias-Groth, , , Jonay I. Gonz´alez Hern´andez , , A. Manchado , , ⋆ Instituto de Astrofis´ıca de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain Dpt. de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain Consejo Superior de Investigaciones Cient´ıficas, Madrid, Spain
Accepted Received In original form
ABSTRACT
We have obtained high resolution optical spectroscopy of 10 reddened O-type starswith UVES at VLT to search for interstellar bands of the naphthalene cation (C H )in the intervening clouds. No absorption features were detected near the laboratorystrongest band of this cation at 6707 ˚A except for star HD 125241 (O9 I). Additionalbands in the optical spectrum of this star appear to be consistent with other transitionsof this cation. Under the assumption that the bands are caused by naphthalene cationswe derive a column density N Np + = (1.2 ± cm − similar to the columndensity claimed in the Perseus complex star Cernis 52 (Iglesias-Groth et al. 2008).The strength ratio of the two prominent diffuse interstellar bands at 5780 and 5797˚A suggests the presence of a σ -type cloud in the line of sight of HD 125241. Key words:
ISM:molecules—ISM:lines and bands—ISM:abundances
The detection of discrete infrared emission bands near 3.3,6.2, 7.7, 8.6, 11.3 and 12.7 µ m, in dusty environments ex-cited by UV photons led to the suggestion that polycyclicaromatic hydrocarbons (PAHs) were present in the interstel-lar medium (L´eger & Puget 1984, Allamandola et al. 1985).These infrared bands are due to C-C and C-H stretching andbending vibrations in an aromatic hydrocarbon material.Since these bands mostly probe specific chemical bonds andnot any particular molecular structure, they cannot proivdeunambiguous identification of single PAHs. The naphtha-lene cation (C H ) is the most simple PAH and one ofthe best characterized spectroscopically in low-temperaturegas phase at laboratory (Pino et al. 1999, Romanini et al.1999). The laboratory characterization, crucial for a poten-tial identification in the interstellar medium, shows that thestrongest optical band of the naphthalene cation is locatedat 6707.4 ˚A with a full width at half maximum (FWHM) ofapprox. 12 ˚A. Progressively weaker bands of similar widthhave been measured at 6488.9, 6125.2 and 5933.5 ˚A(Biennieret al. 2003). Iglesias-Groth et al. (2008) reported the detec-tion of weak absorption ( less than 1.5 % of the continuum)broad optical bands in the spectrum of the star Cernis 52(A3 V, Cernis 1993) which appear to be consistent withthe measured laboratory bands of the naphthalene cation.Gonz´alez Hern´andez et al. (2009) show that the detectedbands are too broad to be originated in the photosphere ofthis star. ⋆ E-mail: [email protected]
Cernis 52 is located behind or embedded in a molecularcloud in the Perseus star forming complex (one of the nearestto the Sun) that causes moderate extinction (A V = 3 mag).This cloud presents millimetric emission (Enoch et al. 2006)and anomalous microwave (10-70 GHz) emission (Watsonet al. 2005) which can be caused by electric dipole radiationof fast spinning PAHs in the intervening cloud (Draine andLazarian 1998, Planck collaboration 2011m). It is possiblethat this anomalous microwave emission region in Perseushas an enhanced abundance of PAHs with respect to thediffuse interstellar medium or that the feature is of circum-stellar origin and therefore that we are considering a ratherpeculiar star. Detection of bands of the naphthalene cationin other lines of sight could show whether this is indeed anexceptional case. Alternatively, if this is a common feature ininterstellar clouds, the hypothesis that some of the ubiqui-tuous diffuse interstellar bands (DIBs) are caused by PAHswould be reinforced. So far there is no firm identification ofa PAH as responsible of DIBs (Sarre 2006).With the goal to investigate the presence of naphthalenecations in diffuse clouds we have obtained optical spectraof high resolution and high signal-to-noise of 10 early-typestars with E(B-V) in the range 0.5 to 1.7 (see Table I). Thereis no report of anomalous microwave emission for any ofthe lines of sight of the stars in our sample. We detect thepresence of broad absorption features in star HD 125241with wavelengths and widths consistent with those of thestrongest optical bands of the naphthalene cation. Upperlimits to the strength of these bands are set for the otherlines of sight. c (cid:13) S. Iglesias-Groth
Wavelength (Angstrom) −2−1.5−1−0.500.511.522.533.5 R e l a t i v e I n t en s i t y Figure 1.
Spectra of the sample stars in the region of the interstellar Na I doublet
Table 1.
Basic data of our programme stars.
Star Spectral type E(B-V) V S/NHD114213 B1Ib 1.11 8.97 690HD125241 09Iab 0.76 8.23 670HD142468 B0.5IB2 0.78 7.88 530HD147889 B2III,IV 1.03 7.90 520HD157038 B1/B2Ia 0.81 6.71 560HD159176 O6V 0.36 5.68 224HD166734 07.5If 1.36 8.42 500HD167971 08,09f 1.04 7.46 250HD168607 B9Iap 1.65 8.28 400HD176162 B4V 0.11 5.5 550
The data presented here are based on observations con-ducted with the UV-Visual Echelle Spectrograph (UVES)fed by the VLT (Kueyen unit) of the ESO Paranal Obser-vatory, Chile. The observed stars, their V magnitude, theirE(B-V) colour excesses, and the final signal-to-noise ratio(S/N) measured near 6750 ˚A, are listed in Table 1. Thespectral range was set to investigate the presence of thefour strongest optical bands of the naphthalene cation. Eachexposure provided nearly complete spectral coverage fromabout 5000 to 7000 ˚A, at a resolving power R ∼ c (cid:13) , 1–, 1–
The data presented here are based on observations con-ducted with the UV-Visual Echelle Spectrograph (UVES)fed by the VLT (Kueyen unit) of the ESO Paranal Obser-vatory, Chile. The observed stars, their V magnitude, theirE(B-V) colour excesses, and the final signal-to-noise ratio(S/N) measured near 6750 ˚A, are listed in Table 1. Thespectral range was set to investigate the presence of thefour strongest optical bands of the naphthalene cation. Eachexposure provided nearly complete spectral coverage fromabout 5000 to 7000 ˚A, at a resolving power R ∼ c (cid:13) , 1–, 1– ?? search for naphthalene in diffuse interstellar clouds Table 2.
Equivalent widths (m˚A) of diffuse interstellar bands for the star sample.Note:Errors are indicated in parenthesis
Star 5780 5797 6113 6196 6203 6270 6284 6376 6379 6613HD114213 332 (3) 111 (5) 13 (1) 39 (2) 70 (4) 42 (5) 540 (60) 31 (2) 52 (2) 157(5)HD125241 526 (10) 109 (5) 9 (1) 56 (2) 131 (5) 107 (5) 1086 (90) 45 (2) 69 (2) 208 (4)HD142468 449 (10) 115 (5) 14 (1) 55 (2) 75 (4) 87 (5) 1176 (90) 25 (2) 70 (2) 185 (5)HD147889 347 (8) 144 (5) 12 (1) 39 (2) 80 (5) 25 (4) 389 (50) 60 (2) 86 (2) 185 (5)HD157038 413 (8) 93 (5) 7 (1) 45 (2) 200 (8) 74 (5) 1147 (90) 26 (2) 136 (4) 161 (5)HD159176 161 (5) 41 (4) 7 (2) 25 (2) 31 (3) 25 (4) 224 (30) 7 (1) 29 (2) 66 (4)HD166734 687 (15) 250 (8) 48 (3) 89 (4) 227 (8) 181 (8) 1209 (90) 99 (8) 220 (2) 398 (7)HD167971 530 (20) 149 (5) 31 (3) 16 (2) 100 (8) 122 (6) 1073 (95) 45 (3) 92 (4) 241 (9)HD168607 796 (36) 273 (8) 25 (2) 72 (3) 149 (5) 134 (6) 1398 (95) 57 (2) 152 (4) 342 (8)HD168625 818 (25) 209 (8) 37 (3) 88 (4) 146 (5) 185 (7) 993 (80) 88 (3) 191 (3) 434 (7) W ( m A ) − T ho r bu r n e t a l . HD147889HD166734HD167971
Figure 2.
A comparison of DIB equivalent widths in this work and in Thorburn et al. (2003).
In Table 2 we report equivalent widths (W) for a set of wellknown DIBs present in the spectra of our stars. We usedthe IRAF splot task to either fit a Gaussian profile to theband (whenever this was suitable) or to integrate the bandwith respect to the pseudocontinuum. The uncertainties ofthe equivalent widths were estimated with the same task,taking into account the S/N in the relevant region of thespectrum and the uncertainty in the location of the contin-uum and in the shape of the band. In general these errors(listed in Table 2 are larger than those derived from the for-mula ∆( W )= 1.064 x FWHM / (S/N) (see e.g. Hobbs etal. 2009) which strictly refers to a band of Gaussian shape.We tested our error determination measuring separately inaveraged subsets of spectra for each star and calculating therms deviation of the equivalent widths. High resolution spec- troscopy for several stars in our sample is available in theliterature and DIB equivalent widths have been previouslyreported (see e.g. Thorburn et al. 2003). A comparison withprevious measurements for our stars it is made in Fig. 2.In general good agreement is found for most of the DIBswith the largest differences in W being of order 20 % for thestrongest and broadest bands where errors are dominatedby the uncertainty in the location of the continuum.In Fig. 3 we plot the final spectra in the region of thestrongest optical band of the naphthalene cation. The zero-point of the wavelength scale adopted here is set by assign-ing the DIB wavelengths listed by Hobbs et al. (2008) to theDIBs detected in our spectra. No obvious broad absorptionsare detected at the wavelength of the strongest optical bandof the naphthalene cation (6707 ˚A), except in the case ofHD 125241 where a broad absorption appears to be presentfrom approximately 6700 to 6720 ˚A. HD 125241 is classified c (cid:13) , 1– ?? S. Iglesias-Groth wavelength (Angstrom)
HD125241HD114213HD142468HD147889HD157038 HD176162HD168607HD167971HD168625HD159176
Figure 3.
Set of spectra in the range of the most intense band of the naphthalene cation. Notice the presence of a band centered around6708 A in the star HD125241 as spectral type O9. We note the similarity of the spectra ofHD 125241 and HD 167971 (spectral type O8-9) also plot-ted in Fig. 3. In the spectral range of this figure, the mostremarkable features in common between these two stars are:the He II 6683 absorption band and a broad absorption at6736 ˚A whose origin is not well established. There are alsoweaker features in both stars associated with the DIBs at6699 and 6729 ˚A and the SI IV emission line at 6701 ˚A.HD 167971 has been extensively studied in the literature,Thorburn et al. (2003) provide equivalent widths for numer-ous narrow DIBs in this star which compare well in strengthwith the values listed in Table 2. If the feature at ∼ ± ∼ c (cid:13) , 1–, 1–
Set of spectra in the range of the most intense band of the naphthalene cation. Notice the presence of a band centered around6708 A in the star HD125241 as spectral type O9. We note the similarity of the spectra ofHD 125241 and HD 167971 (spectral type O8-9) also plot-ted in Fig. 3. In the spectral range of this figure, the mostremarkable features in common between these two stars are:the He II 6683 absorption band and a broad absorption at6736 ˚A whose origin is not well established. There are alsoweaker features in both stars associated with the DIBs at6699 and 6729 ˚A and the SI IV emission line at 6701 ˚A.HD 167971 has been extensively studied in the literature,Thorburn et al. (2003) provide equivalent widths for numer-ous narrow DIBs in this star which compare well in strengthwith the values listed in Table 2. If the feature at ∼ ± ∼ c (cid:13) , 1–, 1– ?? search for naphthalene in diffuse interstellar clouds NII SiIV SiIV NIIHeIHeIIHeII OI?
DD D DD D D
Np+?
D DD DD D D D
Np+?
D D DD D D
Np+? Np+?
D D D D D DD DD D
Wavelength (Angtroms) D R e l a ti v e I n t e n s it y DDD
Figure 4.
Spectrum of HD 125241 in the regions of the four main optical bands of the naphthalene cation (positions indicated by elipses).Known DIBs in the vicinity of these bands are marked with ”D”. Very narrow absorptions are due to telluric lines. Broad photosphericfeatures possibly due to atomic transitions are also indicated. similar spectral type to that of HD 125241 from Tuairisg etal. (2000). The only relevant photospheric feature is an ab-sorption precisely at 6702 ˚A, we argue that this feature mayappear in emission in our star due to different physical condi-tions in its atmosphere. We reproduce the emission featuresat 6701 and 6712 ˚A using Gaussians. The latter emissionline, of uncertain origin, is also seen in the spectrum of thecomparison star plotted at the bottom of Fig. 5. We also plota synthetic DIB spectrum using the list provided by Hobbset al. (2008) and scaling the features in strength to matchthose observed in our spectrum. The features are scaled in-dividually to obtain a best fit to the observed DIBs but wenote that a similar scaling is required for most of them. Allthe DIBs in this spectral range are narrow (FWHM χ -squaredof 1.06). The total equivalent width of this tentative cation band would be W(6707)= 240 ± ∼ c (cid:13) , 1– ?? S. Iglesias-Groth R e l a t i v e I n t en s i t y HD125241 HD176162
Figure 5.
Spectrum of HD 125241 in the region of the strongest band of the naphthalene cation (black solid line). A model (redsolid line) is superimposed to the spectrum. This model combines a synthetic DIB spectrum (dashed green line), a synthetic stellarphotospheric spectrum (dotted blue line), an amission spectrum probably coming from an estended atmosphere (dashed-dotted greenline)and a Gaussian of FWHM=12 ˚A describing the potential contribution of the naphthalene cation strongest optical band at 6707.4˚A (red dashed line). For clarity each of these three contributions are plotted shifted by 0.01 in the vertical axis. At the bottom, thespectrum of star HD 176162 is plotted for comparison. χ -squared of 1.06) the observations in this spectral range.In order to study the presence of the third strongestband of naphthalene, at the middle panel of Fig. 5 we plotour prediction for this band scaling from the 6707 ˚A band(red dashed line) as indicated above. After combining withthe known DIBs (green dashed line) and the stellar pho-tospheric spectra (dotted blue line) we compare with theobservations (red solid line). There are no significant stel-lar photospheric bands in this spectral range, neither strongDIBs. Two narrow DIBs are clearly seen at 6113 and 6117˚A which are helpful to set precisely the wavelength scale ofinterstellar absoprtions. We produce a synthetic DIB modelusing the information in Hobbs et al. (2008) which repro-duces well the two narrow DIBs. The observed spectrum canbe reproduced by a combination of these DIBS with two newbroad band features at 6113 and at 6124 ˚A (with reduced χ -squared of 1.9). The latter coincides in wavelength andintensity with the values expected for the third band of thenaphthalene cation.Finally, at the bottom panel of Fig. 5. we investigatethe presence of the fourth and weakest naphthalene cationband at 5934.5 ˚A. This is a band particularly difficult toaddress since the maximum depth we expect is about 0.2 % of the continuum. As in the other panels of the figure weplot the synthetic photospheric spectrum, the DIB syntheticspectrum and the predicted scaled naphthalene cation band.The combination of the three leads to the continuous redline. The model does provide a poor fit according to the χ -squared value. A broad absorption band seems to be presentin the observed spectrum at the expected location of thefourth naphthalene band, so the presence of naphthalene ispossible, but other additional bands of unknown nature arelikely contributing to the strength of the observed feature.In summary, the characteristics of the absorption fea-tures found in the spectral ranges of the four strongest op-tical bands of the naphthalene cation are consistent withthe presence of these molecules in the intervening cloud inthe line of sight of HD 125241. Adopting for the oscillatorstrength of the transition at 6707.4 ˚A a value of f=0.05(Pino et al. 1999) and using the measured equivalent widthof the band, we derive as in Iglesias-Groth et al. (2008) acolumn density of N Np + ≃ (1.2 ± cm . This is avery similar value to that found in the line of sight of Cernis52. For the remaining stars in our sample we can set upperlimits to the column density which vary from one star toother depending on the S/N in the relevant spectral region.As listed in Table 1 most of the stars have S/N of order 500.This means that bands at 6707 ˚A with maximum depth 1 c (cid:13) , 1–, 1–
Spectrum of HD 125241 in the region of the strongest band of the naphthalene cation (black solid line). A model (redsolid line) is superimposed to the spectrum. This model combines a synthetic DIB spectrum (dashed green line), a synthetic stellarphotospheric spectrum (dotted blue line), an amission spectrum probably coming from an estended atmosphere (dashed-dotted greenline)and a Gaussian of FWHM=12 ˚A describing the potential contribution of the naphthalene cation strongest optical band at 6707.4˚A (red dashed line). For clarity each of these three contributions are plotted shifted by 0.01 in the vertical axis. At the bottom, thespectrum of star HD 176162 is plotted for comparison. χ -squared of 1.06) the observations in this spectral range.In order to study the presence of the third strongestband of naphthalene, at the middle panel of Fig. 5 we plotour prediction for this band scaling from the 6707 ˚A band(red dashed line) as indicated above. After combining withthe known DIBs (green dashed line) and the stellar pho-tospheric spectra (dotted blue line) we compare with theobservations (red solid line). There are no significant stel-lar photospheric bands in this spectral range, neither strongDIBs. Two narrow DIBs are clearly seen at 6113 and 6117˚A which are helpful to set precisely the wavelength scale ofinterstellar absoprtions. We produce a synthetic DIB modelusing the information in Hobbs et al. (2008) which repro-duces well the two narrow DIBs. The observed spectrum canbe reproduced by a combination of these DIBS with two newbroad band features at 6113 and at 6124 ˚A (with reduced χ -squared of 1.9). The latter coincides in wavelength andintensity with the values expected for the third band of thenaphthalene cation.Finally, at the bottom panel of Fig. 5. we investigatethe presence of the fourth and weakest naphthalene cationband at 5934.5 ˚A. This is a band particularly difficult toaddress since the maximum depth we expect is about 0.2 % of the continuum. As in the other panels of the figure weplot the synthetic photospheric spectrum, the DIB syntheticspectrum and the predicted scaled naphthalene cation band.The combination of the three leads to the continuous redline. The model does provide a poor fit according to the χ -squared value. A broad absorption band seems to be presentin the observed spectrum at the expected location of thefourth naphthalene band, so the presence of naphthalene ispossible, but other additional bands of unknown nature arelikely contributing to the strength of the observed feature.In summary, the characteristics of the absorption fea-tures found in the spectral ranges of the four strongest op-tical bands of the naphthalene cation are consistent withthe presence of these molecules in the intervening cloud inthe line of sight of HD 125241. Adopting for the oscillatorstrength of the transition at 6707.4 ˚A a value of f=0.05(Pino et al. 1999) and using the measured equivalent widthof the band, we derive as in Iglesias-Groth et al. (2008) acolumn density of N Np + ≃ (1.2 ± cm . This is avery similar value to that found in the line of sight of Cernis52. For the remaining stars in our sample we can set upperlimits to the column density which vary from one star toother depending on the S/N in the relevant spectral region.As listed in Table 1 most of the stars have S/N of order 500.This means that bands at 6707 ˚A with maximum depth 1 c (cid:13) , 1–, 1– ?? search for naphthalene in diffuse interstellar clouds R e l a t i v e I n t en s i t y R e l a t i v e I n t en s i t y R e l a t i v e I n t en s i t y Figure 6.
Spectrum of star HD 125241 (solid black line) in theregions of the 6488.7, 6123.5 and 5934.5 ˚A naphthalene cationbands (top, middle and bottom panels). In each panel we plot amodel (red solid line) superimposed to the spectrum. Each modelis the combination of three contributions: DIB synthetic spectrum(dashed green line), a stellar photospheric spectrum (dotted blueline) and a Gaussian of FWHM=10-12 ˚A describing the potentialcontribution of the relevant naphthalene cation band (red dashedline). In each panel, these individual contributions to the modelare plotted shifted in the vertical axis for clarity. % would have been detected in most cases with high levelof confidence. We set upper limits of N Np + cm for the intervening clouds in the other lines of sight.The extinction in HD 125241 is similar to that of Cer-nis 52, but the equivalent widths and profiles of the 5780and 5797 DIBs in HD 125241 (see Fig. 6) indicate that theintervening cloud is of σ type while in Cernis 52 is of ζ -type.It is important to carry out a detailed characterization ofthe physical parameters and molecular material in this newcloud. In the case of the intervening cloud towards Cernis52, spectroscopy of the C bands (Iglesias-Groth 2010a) andCH, CH + (Iglesias-Groth et al. 2010b) indicate a rather highabundance of these molecules. The physical and chemicalconditions may be suitable for formation of PAHs. Unfor-tunately very little is known about the cloud in HD 125241and CH and CH + transitions are not covered by our spectraneither the strongest series of C . Only the (4,0) Phillipsseries is present in our spectrum but the transitions are tooweak preventing any reliable measurement of the gas kinetictemperature.Interestingly, there are four clouds identified usingSpitzer images in the field of the star HD 125241, they arelocated at angular distances of ∼ ∼ We have searched for the strongest optical transition of thenaphthalene cation at 6707 ˚A in the spectra of 10 reddened c (cid:13) , 1– ?? S. Iglesias-Groth
HD125241HD176162
Figure 7.
DIBs at 5780 and 5797 ˚A in the spectra of stars HD 125241 (solid line) and HD 176162 (dotted line).We shown relativeintensity against of the wavelength (Angstroms)
O and B type stars. We find evidence for a broad absorptionwhich could be associated to this band only in the case ofthe O9 I star HD 125241 which displays a broad absorptionband at 6707 ˚A consistent in wavelength and FWHM withlaboratory measurements of the strongest optical band ofthe naphthalene cation. Weaker bands of this cation seemto be present in the spectrum of this star with consistentstrength at 6489, 6125 and 5934 ˚A. Assuming that thesebands are indeed caused by the naphthalene cations we de-rive a column density of N Np + = (1.2 ± cm .The diffuse interstellar bands at 5780 and 5797 support thepresence of a sigma-type cloud in the line of sight of thisstar.In the other stars of the sample we set upper limits tothis column density a factor 2-3 lower in spite of the higherextinction associated with several of the intervening clouds.Additional studies of the intervening cloud and circumstel-lar environment in HD 125241 may provide valuable infor-mation on the physical and chemical conditions that maygovern an active formation of PAHs. c (cid:13) , 1–, 1–