faint emission lines in the blue and red spectral regions of the night airglow

Faint Emission Lines in the Blue and Red Spectral Regions of the Night AirglowAuthor(s): Donald E. Osterbrock, Richard T. Waters, Thomas A. Barlow, Tom G. Slanger, andPhilip C. CosbySource: Publications of the Astronomical Society of the Pacific, Vol. 112, No. 771 (May 2000),pp. 733-741Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/316568 .

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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 112 :733È741, 2000 May2000. The Astronomical Society of the PaciÐc. All rights reserved. Printed in U.S.A.(

Faint Emission Lines in the Blue and Red Spectral Regions of the Night Airglow1

DONALD E. OSTERBROCK AND RICHARD T. WATERS

University of California Observatories/Lick Observatory, Department of Astronomy and Astrophysics,University of California, Santa Cruz, CA 95064 ; don=ucolick.org, waters=ucolick.org

THOMAS A. BARLOW

Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125 ; tab=ipac.caltech.edu

AND

TOM G. SLANGER AND PHILIP C. COSBY

Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025 ; slanger=mplvax.sri.com, cosby=mplvax.sri.com

Received 1999 December 12 ; accepted 2000 February 7

ABSTRACT. Co-added night-sky spectra, obtained as by-products of exposures with the Keck I 10 mtelescope on Mauna Kea and the HIRES high-resolution echelle spectrograph over a period of approx-imately 4 years, all completely independent of similar data published earlier, are presented. The new datatotal over 150 hours exposure in one order (5505È5625 more than 100 hours in 16 orders, and more thanA� ),50 hours in all orders in the spectral range 3923È7853 and include smaller numbers of hours over the entireA�range 3618È9023 From these data, co-added in the red region to the previously published data, twoA� .additional Meinel OH bands, 8È1 and 7È0, were found in emission in the spectrum of the night airglow, thepresence of the 6È0 band was conÐrmed, and numerous lines of the 10È4 and 10È5 bands were detected.Three Hg I light-pollution lines were detected as weakly present in the Mauna Kea night sky. Three otherpredicted Meinel bands are too faint and still were not detected with certainty. Similarly, neither OD nor theRb I or Cs I resonance lines were seen. Upper limits were set on the latter, which are consistent with theirabundance ratios to K and the observed strength of the K I resonance line j7699, if the excitation mecha-nisms of all these three alkali atoms were equally e†ective. Brief references are given to other papers in pressbased on these new spectral data and to other work in progress on identiÐcations of many additional O2bands in the spectrum of the night airglow. A table summarizes all the identiÐcations of all OH emissionbands in the spectrum of the night airglow, in all spectral regions.

1. INTRODUCTION

The high-resolution echelle spectrograph (HIRES) on theKeck I 10 m telescope automatically records excellentnight-sky spectra as a by-product of every exposure on astar or quasar. In previous papers these spectra have beenused to prepare a high-resolution spectral atlas of the OHMeinel (vibration-rotation) bands, making it possible forobservers to easily Ðnd lines with accurately known wave-lengths for accurate wavelength calibrations (Osterbrock etal. 1996, hereafter Paper I) and to identify many weak OHfeatures including satellite lines and high rotational levellines (Osterbrock, Fulbright, & Bida 1997, hereafterPaper II), 18OH and 17OH isotopic bands, and a few lines

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 Lick Observatory Bulletin 1391. Based on observations obtained at

the W. M. Keck Observatory, which is operated by the California Instituteof Technology and the University of California.

from OH (l@\ 10), not excited by the primary mechanism(Osterbrock et al. 1998, hereafter Paper III). These spectrahave also proved most useful in identifying many weakerlines in the night airglow as molecular lines in severalO2di†erent band systems, which can be followed to quite highvibrational quantum numbers (Slanger et al. 1997 ; Slanger& Osterbrock 1998). Detection and accurate measurementof faint lines demand high signal-to-noise ratios, which wereobtained by co-adding many night-sky spectra taken bydi†erent observers, for total exposures ranging up to morethan 100 hours in quite a few echelle orders (Paper III).These co-added spectra were conÐned to the spectral region5720È8932 in which the strongest OH lines fall, extend-A� ,ing longward to the wavelength region in which the sensi-tivity of the CCD used in HIRES is decreasing rapidly. Wehave now accumulated a good deal more spectral data inthat ““ red ÏÏ spectral region, and a similar amount in the““ blue ÏÏ region down to 3981 as well as smaller amountsA� ,

733



734 OSTERBROCK ET AL.

down to 3618 The limits of the available data were set byA� .the wavelength regions which the HIRES observers use instudying stars and quasars ; all of our night-airglow spectraare by-products of such observations. In this paper the newdata are described and listed, and applications for detectingand measuring (or setting upper limits to) additional faintOH bands and lines, faint Hg I sky-pollution lines, alkali-metal resonance lines, H I lines, and lines are discussedO2or summarized.

2. OBSERVATIONAL DATA

All the spectra used in this paper were taken with HIRESon the Keck I 10 m telescope on Mauna Kea, Hawaii, at analtitude 4160 m, latitude ]20¡. HIRES is described morefully in Paper I and references therein. The slit width, CCD

detector, Th-Ar hollow-cathode tube, extraction of the skyspectrum, and division by Ñat-Ðeld spectra were asdescribed in Paper III. In the reduction process applied tothe spectra in that paper and in this one, the primary wave-length calibration was provided by the Th-Ar tube, but thenan overall shift in pixel space, determined by a global Ðt tothe wavelengths of a large number of selected sky lines, gavethe Ðnal zero point of the wavelengths. Shifts up to nearly 1pixel (2.1 km s~1, or about 0.05 in the red spectral region),Óhave been measured and corrected. The months in whichthe spectra used in this paper were taken, and the observersÏnames, are listed in Tables 1 (blue region) and 2 (red). Thesespectra were collected at Caltech and University of Califor-nia, San Diego, were checked, their headers were completed,and they were then transmitted to University of California,Santa Cruz, where they were co-added, using the pro-cedures described in Paper III. There appears to be very

TABLE 1

JOURNAL OF OBSERVATIONS : BLUE CO-ADDED SPECTRUM

Date Observers

1993 Nov . . . . . . W. L. W. Sargent, J. K. McCarthy1994 Jun . . . . . . . W. L. W. Sargent, D. S. Womble, I. N. Reid1994 Oct . . . . . . . W. L. W. Sargent, D. S. Womble, L. Lu1995 Feb . . . . . . W. L. W. Sargent, T. A. Barlow1995 Jun . . . . . . . W. L. W. Sargent, T. A. Barlow, J. Jugaku1995 Sep . . . . . . . W. L. W. Sargent, T. A. Bida1995 Dec . . . . . . F. W. Hammann, V. T. Junkkarinen1996 Jan . . . . . . . W. L. W. Sargent, T. A. Barlow, M. Takada-Hidai, V. T. Junkkarinen, R. W. Lyons1996 Feb . . . . . . F. W. Hammann, V. T. Junkkarinen1996 Aug . . . . . . F. Cha†ee1996 Sep . . . . . . . D. R. Tytler1997 Mar . . . . . . W. L. W. Sargent, M. Rauch1997 Apr . . . . . . A. M. Wolfe1997 Sep . . . . . . . J. X. Prochaska, T. A. Barlow, W. L. W. Sargent, M. Rauch1997 Oct . . . . . . . J. X. Prochaska, T. A. Barlow1997 Nov . . . . . . F. Cha†ee1998 Feb . . . . . . D. A. Kirkman, D. P. Tytler, J. X. Prochaska1998 May . . . . . . D. A. Kirkman, E. L. Lever

TABLE 2

JOURNAL OF OBSERVATIONS : RED CO-ADDED SPECTRUM

Date Observers

1993 Nov . . . . . . W. L. W. Sargent, J. K. McCarthy1994 Jun . . . . . . W. L. W. Sargent, D. S. Womble, I. N. Reid1994 Oct . . . . . . W. L. W. Sargent, D. S. Womble, L. Lu1995 Feb . . . . . . W. L. W. Sargent, T. A. Barlow1995 Jun . . . . . . W. L. W. Sargent, T. A. Barlow, J. Jugaku1995 Sep . . . . . . W. L. W. Sargent, T. A. Bida1995 Dec . . . . . . F. W. Hammann, V. T. Junkkarinen1996 Jan . . . . . . . W. L. W. Sargent, T. A. Barlow, M. Takada-Hidai, V. T. Junkkarinen, R. W. Lyons1996 Feb . . . . . . F. W. Hammann, V. T. Junkkarinen1996 Aug . . . . . . F. Cha†ee1997 Mar . . . . . . W. L. W. Sargent, M. Rauch1997 Sept . . . . . . W. L. W. Sargent, M. Rauch

2000 PASP, 112 :733È741



FAINT EMISSION LINES IN NIGHT AIRGLOW 735

little loss of resolution introduced in the co-adding process ;the wavelengths of strong OH lines measured on them aretypically correct to 0.001 The two summed spectraÓ.resulting from this process may then be described as asecond ““ red ÏÏ spectrum, covering the same wavelengthrange as the co-added spectrum of Paper III, and a new““ blue ÏÏ one. The total exposure time in each echelle orderand the wavelength range included in it are listed in Tables3 (blue) and 4 (red). As described in Paper III, these expo-sure times apply to the middle part of each order, but theends of each order generally received less exposure, becausedi†erent observers used di†erent settings of the echelle. Cor-respondingly, however, the wavelengths near the ends ofeach order are also included at the opposite ends of theorders just above or below them. Note that the wavelengthranges included in the various echelle orders in the red

TABLE 3

ECHELLE ORDERS, WAVELENGTHS, AND EXPOSURE

TIMES : BLUE CO-ADDED SPECTRUM

Wavelength Range Exposure TimeOrder (A� ) (hr)

63 . . . . . . 5592È5714 134.964 . . . . . . 5505È5625 158.565 . . . . . . 5420È5538 132.566 . . . . . . 5338È5454 135.367 . . . . . . 5258È5373 141.668 . . . . . . 5181È5294 131.969 . . . . . . 5106È5217 123.870 . . . . . . 5033È5143 126.671 . . . . . . 4962È5070 143.072 . . . . . . 4893È5000 144.573 . . . . . . 4826È4928 102.774 . . . . . . 4761È4861 96.475 . . . . . . 4698È4849 98.476 . . . . . . 4636È4733 80.077 . . . . . . 4576È4672 91.178 . . . . . . 4517È4612 82.979 . . . . . . 4460È4553 81.780 . . . . . . 4404È4496 89.381 . . . . . . 4350È4441 80.982 . . . . . . 4297È4386 74.983 . . . . . . 4245È4334 73.584 . . . . . . 4194È4282 77.385 . . . . . . 4145È4230 79.086 . . . . . . 4097È4181 59.987 . . . . . . 4050È4133 68.588 . . . . . . 4012È4086 73.289 . . . . . . 3967È4039 56.990 . . . . . . 3923È3994 52.091 . . . . . . 3880È3950 12.992 . . . . . . 3838È3907 8.993 . . . . . . 3796È3865 8.794 . . . . . . 3756È3823 6.695 . . . . . . 3719È3783 2.896 . . . . . . 3680È3744 2.497 . . . . . . 3642È3705 1.798 . . . . . . 3618È3667 0.4

TABLE 4

ECHELLE ORDERS, WAVELENGTHS, AND EXPOSURE

TIMES : RED CO-ADDED SPECTRUM

Wavelength Range Exposure TimeOrder (A� ) (hr)

40 . . . . . . 8799È9023 7.941 . . . . . . 8565È8810 40.142 . . . . . . 8361È8601 41.643 . . . . . . 8166È8401 41.644 . . . . . . 7981È8210 42.445 . . . . . . 7804È8028 42.446 . . . . . . 7634È7853 58.547 . . . . . . 7472È7686 63.748 . . . . . . 7316È7526 72.349 . . . . . . 7167È7365 74.050 . . . . . . 7023È7218 74.351 . . . . . . 6886È7076 53.052 . . . . . . 6753È6940 81.353 . . . . . . 6626È6808 80.954 . . . . . . 6503È6683 95.555 . . . . . . 6385È6562 106.456 . . . . . . 6272È6445 71.357 . . . . . . 6176È6331 79.858 . . . . . . 6069È6217 107.259 . . . . . . 5967È6112 132.860 . . . . . . 5867È6010 132.261 . . . . . . 5771È5912 68.662 . . . . . . 5682È5806 133.0

spectrum in this paper are identical with those in Paper III,except for orders 40 and 62, at either end of the spectra. Allthe data in this paper are completely independent of thosein Paper III, so by co-adding the two red spectra we haveformed an even better signal-to-noise ratio co-added spec-trum, with exposure time in each order the sum of thoselisted in the two papers. We used this ““ super ÏÏ spectrum tosearch for the very faint features in the red spectral regiondiscussed in °° 3 and 5.

3. ADDITIONAL OH BANDS

All the OH Meinel bands with l@¹ 9 which fall in the redor near-infrared regions have already been observed withHIRES and reported in Papers I and II and at lowerresolution in earlier papers by other authors. The rotationalenergy levels with low J are very accurately known fromlaboratory spectroscopy (Abrams et al. 1994), and it isstraightforward to predict the wavelengths of the lines ofadditional Meinel bands in the blue spectral region. Theyare expected to be weak, because they have large *l andcorrespondingly small transition probabilities, but our blueco-added spectrum provides the best existing data in whichto seek them. All the OH Meinel bands have similar charac-teristic patterns, in which the line, very near theQ1(1.5)band origin, is the strongest feature, followed by the alter-

2000 PASP, 112 :733È741




FIG

.1.

ÈO

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72of

HIR

ES

co-a

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nigh

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ission

-lin

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8È1

band

2000 PASP, 112 :733È741




nating and weaker lines at longer wavelengths,P1(J) P2(J)with maximum intensity at or The andP1(2.5) P1(3.5). R1

bands, consisting of weaker lines which form heads atR2shorter wavelength, are relatively confused, particularly in aregion in which there are many other lines of comparablestrength, as in the blue spectral region. At Ðrst glance, at thehigh spectral resolution of HIRES the night-airglow spec-trum in this region seems to be very noisy, but in fact itconsists of many weak lines which repeat on each exposure.Many of them are weak lines of several di†erent bandO2systems, now being identiÐed individually by Slanger et al.(1999). It is straightforward to estimate the intensities of thefaint OH Meinel bands in the ““ blue ÏÏ (and violet) spectralregion using the ratios of their transition probabilities to thetransition probabilities of the stronger bands whose abso-lute average intensities are listed by Slanger & Osterbrock(1999). Transition probabilities of the stronger bands areavailable from the calculations of Goldman et al. (1998) andfor the weaker bands from similar calculations described inPaper III. For each band the calculated wavelength of

and the estimated intensity are listed in Table 5.Q1(1.5)These estimates were determined by multiplying the inten-sities of the 7È1, 8È2, and 9È3 bands given by Slanger &Osterbrock (1999) by the ratios of the intensities of thecalculated transition probabilities for transitions with thesame l@. Using this same procedure the intensity of the 9È2band, previously observed but not listed in Table 1 of theirpaper, is estimated to be 7.3 R, the omnidirectional emissioncoefficient in a spectral line or band, integrated along avertical line from the observer on the surface of the Earththrough the entire atmosphere, in units of 106 photonscm~2 s~1 (see, e.g., Chamberlain & Hunten 1987). Thus forinstance an emission coefficient of 1 photon cm~3 s~1extending though a region 10 km thick in the upper atmo-sphere would correspond to an ““ intensity ÏÏ or ““ brightness ÏÏ(we use these terms interchangeably in this paper) to 1 R. Inthe co-added blue spectrum the OH 8È1 band, expected tobe the strongest in this region, has been identiÐed, as shownin Figure 1. Note that, because of the weakness of the lines,and the strength of the ““ continuum,ÏÏ the zero of the inten-sity scale has been o†set, in contrast to the plots of thestronger bands shown in Paper I. The ““ continuum,ÏÏ fairlystrong in the blue spectral region, is at least in part the solar

TABLE 5

OH MEINEL BANDS : BLUE SPECTRAL REGION

Q1(1.5) Wavelength Estimated IntensityBand (A� ) (R)a

8È1 . . . . . . 4905.08 2.07È0 . . . . . . 4642.05 0.39È1 . . . . . . 4420.15 0.48È0 . . . . . . 4174.22 0.069È0 . . . . . . 3817.78 0.01

a 1 R\ 106 cm~2 s~1.

FIG. 2.ÈPortion of HIRES co-added night-sky emission-line spectrum,with simulation (details in text) showing Meinel OH 7È0 band.

spectrum, resulting from scattered moonlight, twilight (insome exposures), and zodiacal light. The strongest solarabsorption features in Figure 1 are Fe I jj4957, 4921 ;several other solar absorption lines can be seen, and stillothers can be found in every echelle order, generally strong-er in the blue and violet spectral regions. In addition, oneother OH Meinel band has been identiÐed by comparisonof the observed spectrum with simulations, calculated withrepresentative temperatures, and intensities scaled by theestimates listed in Table 5. This comparison of the observedand simulated spectra of the 7È0 band is shown in Figure 2.It is clearly present among many other emission features inthe nightglow and the dense solar absorption lines in thezodiacal light and scattered moonlight. For reference, simu-lations for the Chamberlain bands assigned by SlangerO2et al. (1999) in this spectral region are also shown in Figure2. The line, which lies outside the wavelength rangeQ1(1.5)of the Ðgure, is also clearly present. A similar comparisonfor the 9È1 band, although it seems to show some of theexpected lines, does not appear to show others. Furtherevidence is needed. The 8È0 and 9È0 bands are too faint tobe detected in these spectra but, like the 9È1 band, maybecome visible after all the lines have been identiÐed andO2eliminated and the solar absorption lines in the continuumhave been correctly eliminated. Of the bands identiÐed onthe plots of spectra in Paper I, the evidence for the 6È0band, consisting of only the two lines marked there, R1(1.5)and is perhaps the least convincing. We thereforeR2(1.5),show in Figure 3 the and several and lines ofQ1(1.5) P1 P2

2000 PASP, 112 :733È741




FIG

.3.

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2000 PASP, 112 :733È741




this same band. From this spectrum it is clearly present.Table 6 summarizes the information on the Meinel bands inthe blue, red, and near-infrared spectral regions that havebeen measured with HIRES in the night-airglow spectrum.It also includes the Meinel bands observed in the infraredby Oliva & Origlia (1992) in the course of their ground-based measurements with an infrared spectrometer on theESO 3.6 m telescope. In addition it includes the Meinelbands in the far-infrared, from measurements made byDodd et al. (1994) with a liquid heliumÈcooled Michelsoninterferometer fed by a 0.3 m telescope on the space shuttleDiscovery, looking tangentially into the OH-emitting regionat the limb of the Earth. They observed all the fundamentalMeinel bands (*l\ 1) as listed and also the pure rotationalbands (*l\ 0) still further in the infrared with l\ 0È6(which are not listed in the table). It can be seen that allpossible OH Meinel bands with l@¹ 9 have been directlyobserved in the night airglow, except for the three predictedweak bands in the blue spectral regions, and the 9È6 band,which is in the region 13666È14500 heavily obscuredA� ,from the ground by very strong water-vapor bands.H2O

4. Hg I LINES

Although in Paper I it was stated that no sign of Hg I

light-pollution lines was seen in the HIRES spectra taken atMauna Kea, that was based on only 2 hours of data.With the much longer exposure times of the co-added““ blue ÏÏ spectrum of the current paper, three lines are detect-able, two of them unblended. They are Hg I jj5460.74,4358.34, shown in Figure 4, with measured wavelengthsidentical to the laboratory wavelengths to 0.01 A thirdÓ.Hg I line, j4046.56, is also present but blended with aweaker, presently unidentiÐed line. This is the Ðrst detectionof Hg I at Mauna Kea known to us. Although mercurylighting is prohibited by law on the island of Hawaii, suchlights are in operation there. It is also possible that some orall of the light pollution originates in Maui, or the moredistant but more brightly lit Honolulu, on Oahu. The mea-sured photon intensity ratio of Hg I j5461 (in echelle order65) to the night-airglow line [O I] j5577 (order 64) is(1.5^ 0.1)] 10~3, the main uncertainty resulting frommatching the continuum of the two orders. Note that thismeasured ratio represents a long-term average over themonths, bridging more than 4 years, listed in Table 1. Agood mean value of the intensity of [O I] j5577 is 85 R(Slanger & Osterbrock 1999), resulting in a calculated meanintensity 0.13 R for Hg I j5461 at Mauna Kea, for those 4years. For comparison, at Mount Hamilton, from two low-resolution sky spectra recently obtained by R. P. S. Stone

FIG. 4.È(L eft) Hg I j5460.74 in the Mauna Kea night-airglow spectrum. (Right) Same for Hg I j4358.34.

2000 PASP, 112 :733È741




TABLE 6

OH MEINEL BANDS OBSERVED IN NIGHT-AIRGLOW SPECTRA

Upper Vibrational Level l@

Lower Level l@@ 9 8 7 6 5 4 3 2 1

8 . . . . . . . . . . . . . . . . 9È8 (D)7 . . . . . . . . . . . . . . . . 9È7 (O) 8È7 (D)6 . . . . . . . . . . . . . . . . 8È6 (O) 7È6 (D)5 . . . . . . . . . . . . . . . . 9È5 (II)a 8È5 (O) 7È5 (O) 6È5 (D)4 . . . . . . . . . . . . . . . . 9È4 (I)a 8È4 (II)a 7È4 (O) 6È4 (O) 5È4 (D)3 . . . . . . . . . . . . . . . . 9È3 (I)a 8È3 (I)a 7È3 (I)a 6È3 (O) 5È3 (D) 4È3 (D)2 . . . . . . . . . . . . . . . . 9È2 (I)a 8È2 (I)a 7È2 (I)a 6È2 (I)a 5È2 (O) 4È2 (O) 3È2 (D)1 . . . . . . . . . . . . . . . . 8È1 (IV) 7È1 (I)a 6È1 (I)a 5È1 (I)a 4È1 (II, O) 3È1 (O) 2È1 (D)0 . . . . . . . . . . . . . . . . 7È0 (IV) 6È0 (I)a 5È0 (I)a 4È0 (I)a 3È0 (II)a 2È0 (O) 1È0 (D)

NOTE.ÈI \ Paper I ; II \ Paper II ; IV \ this paper ; O \ Oliva & Origlia 1992 ; D \ Dodd et al. 1994.a Broadfoot & Kendall 1968 (low-resolution spectrum).

with the Nickel 1 m telescope, the intensity ratio was j5461/j5577 \ 0.5 near the zenith, and 1.5 to the southwest, overSan Jose, in the early evening. Light pollution by Hg I atLick Observatory is thus stronger by a factor D103 than atMauna Kea, or than it probably was at Mount Hamiltonbefore World War II.

5. OTHER LINES

In addition to the work on OH and Hg I described above,a paper has been prepared, based on these spectra, on theatomic lines of Li I, Na I, and K I in the night airglow. It iscurrently in press (Slanger & Osterbrock 1999). Anotherpaper, on the geocoronal H I lines Ha, Hb, and Hc, all threemeasured in these co-added spectra, is in preparation. AtSRI, very good progress is being made in measuring andidentifying many previously unobserved bands in theO2atmospheric band system to at least(b 1&

g`ÈX 3&

g~)

and in the Chamberlain systemb 1&g`(l\ 15),

to (l\ 12) (Slanger et al. 1999). Rota-(A@ 3*uÈa 1*

g) a 1*

gtional temperatures, showing the characteristic ““ two-temperature ÏÏ distribution have been determined for all thestronger OH bands, from line-intensity measurements onthe ““ super ÏÏ co-added sky spectrum mentioned above(Cosby, Slanger, & Osterbrock 1999). A paper describingthis work and giving the results is in preparation. In PaperIII measurements were reported of a few lines in the 10È4and 10È5 Meinel bands, emitted from the upper vibrationallevel l@ \ 10, which cannot be excited by the primaryprocess e†ective in the night airglow. As stated in thatpaper, these very weak lines show that a second, much lessprobable excitation process also occurs, probably involvingvibrationally excited ozone molecules. Now, with the supersky spectrum, which essentially doubles the exposure timein the region 5720È8932 one of us (P. C. C.) has identiÐedA� ,many more lines in the 10È4 and 10È5 bands, essentially all

the lines to or respectively,P1 P1(5.5) P1(6.5), Q1(1.5),to or respectively, andQ1(2.5), P2(1.5) P2(2.5) P2(3.5),

to or respectively. These identiÐca-R1(1.5) R1(3.5) R1(4.5),tions, and the rotational temperatures for the vibrationallevel l@\ 10, will be described and discussed in the paper inpreparation mentioned above. Since the Na I and K I reso-nance lines are present in the night-airglow spectrum, asmentioned above, we have also searched the super sky spec-trum for the corresponding Rb I and Cs I lines, which mightbe expected to be present also. The very accurately knownwavelengths of these lines are listed in Table 7 ; in each casethe line of shorter wavelength is the line,n 2S1@2Èn 2P3@2expected to be twice as strong as the longer wavelength

None of these four lines was found in then 2S1@2Èn [email protected] sky spectrum. Two of them, one each of Rb I and Cs I,are not well placed for detection, too close to fairly strongOH lines, as stated in Table 7. For the other two we have setupper limits to their strengths, above which we could haveobserved them if present. In each case a very weak feature inthe same order, not at the wavelength of the Rb I or Cs I

line, was measured. For one line it was as faint a feature ascould have been detected if it had been in the position of theline sought, in the other case, it was twice as strong. Anearby OH line was also measured in each order for cali-

TABLE 7

ALKALI-METAL RESONANCE LINES

Wavelength Upper LimitAtom (A� ) (R)a Comparison OH Line

Rb I . . . . . . 7200.23 3 ] 10~2 8È3 R1(3.5) j7238.787947.60 6 ] 10~2 5È1 P2(1.5) j7949.20

Cs I . . . . . . 8521.10 3 ] 10~2 6È2 P2(5.5) j8538.68Blended with OH

8943.50 . . . 7È3 P2(3.5) j8943.40

a 1 R\ 106 cm~2 s~1.

2000 PASP, 112 :733È741




bration through the mean strength (in rayleighs), listed bySlanger & Osterbrock (1999). From this, and a calculationof the relative strengths of all the lines in that band, usingthe rotational temperatures derived as stated above, thestrength (in rayleighs) of each OH line in that band isknown. The resulting upper limits to the Rb I and Cs I linesare listed in Table 7. For Rb I, the upper limit to thestrength of j7200, 3 ] 10~2 R, implies an upper limit to theweaker j7948, 1.5 ] 10~2 R, more severe than the upperlimit directly estimated. For comparison, the observedstrength of K I j7699 (the longer wavelength component ofthe corresponding doublet) is 1.0 R (Slanger & Osterbrock1999). The ratio of abundances (by number of nuclei) of Rbto K in meteorites is 1.9] 10~3 (Anders & Grevesse 1989),and if this same ratio is applied in the upper atmosphereand if the excitation mechanism in these two atoms had thesame rates, the strength of Rb I j7699 would be expected tobe only 1.9] 10~3 R, well below the upper limit. Thepresent observation is therefore not very restrictive. Theabundance ratio in meteorite Cs/K is smaller, 1] 10~4,and the expected strength of Cs I j8521 under these sameconditions would be only 2 ] 10~4 R, much smaller thanthe observational upper limit. Finally, lines of the isotopicOD molecule were searched for in the super sky spectrum,using the predicted wavelengths and procedures outlined inPaper III. No convincing evidence for any OD band wasfound. Probably sky spectra taken further in the infrared,where the OD bands are predicted to be much stronger,

obtained perhaps with an advanced high-resolution astron-omical spectrograph such as NIRSPEC, now being put intooperation at the Keck II telescope, will be needed to detectOD in the night-airglow spectrum (Figer et al. 1999).

6. CONCLUSION

These very long-exposure co-added spectra have revealedmany spectral lines previously unobserved in the nightairglow and have provided upper limits to the strength ofothers, still unobserved. Similar spectra further in the infra-red will probably be required to identify OD and other lessabundant diatomic molecules.

We are most grateful to Steven S. Vogt, Joseph S. Miller,and Fred Cha†ee for their encouragement and assistance ingetting and using the Keck HIRES data and to all theobservers listed in Tables 1 and 2 who were so generous inmaking their sky spectra available to us. We are also grate-ful to Jon P. Fulbright for many valuable discussions on themethods for combining and handling spectral data frommany observers and to Robert W. Goodrich, RobertMcLaren, and Richard Wainscoat for their comments onthe possible sources of light pollution at Mauna Kea. Last,we wish to thank the National Science Foundation forpartial support of this research under grants AST 91-23547and ATM 94-11600.

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faint emission lines in the blue and red spectral regions of the night airglow

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