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atomic resonance/ fluorescence lines may be linearly polarized when anisotropic optical pumping produces an anisotropic angular momentum distribution ( `` alignment '' ) within the ground state @xcite ( hereafter yl06,yl07,yl08 ) .
this occurs if the pumping photon rate is greater than the collision rate , for instance within 1 - 10 pc of an ob star . in the presence of a magnetic field , `` magnetic realignment '' then occurs when the larmor frequency is greater than the photon rate . for the `` saturated '' case ( valid in the diffuse ism , where any field @xmath0 @xmath1gauss causes realignment ) , the polarization depends on the 3d geometry of the magnetic field , the ion ground state configuration and sometimes on the pumping spectrum . in circumstellar matter , @xmath2 @xmath1gauss
is required for realignment , and the polarization can depend also on the field strength .
related effects have been seen in the sun @xcite , but have not yet been demonstrated elsewhere . as a diagnostic of the magnetic field ,
magnetic realignment is potentially more powerful than the 21 cm zeeman effect , since it is sensitive to weaker fields and works in hot gas , and more powerful than dust alignment , since it is sensitive to 3d geometry , gas properties and velocity .
this paper explores observational aspects of this effect .
figure 1 shows the observational geometry .
a gas cloud with a magnetic field at an angle @xmath3 from the line of sight is illuminated by a nearby star ( the `` pumping star '' ) at an angle @xmath4 from the field ( @xmath5 from the line of sight ) . in the absorption case ,
flux from a star behind the cloud ( the `` target star '' , which may be the same as the pumping star ) is absorbed by ions in the cloud .
if the ions are aligned , the resultant absorption line is linearly polarized , with a position angle either parallel or perpendicular to the projected magnetic field , and a degree that depends on @xmath3 and @xmath4 . for absorption , yl06
show that a non - zero polarization requires ions with at least three fine states in the ground level @xmath6 . in neutral gas , the most common ions are ni , oi , sii , and feii . the spectral resolution required is @xmath7 , to resolve the interstellar lines .
since the absorption lines are almost entirely in the fuv , this requires a high resolution fuv spectropolarimeter @xcite .    in the emission case ,
the observed light is light from the pumping star which is scattered into the line of sight .
the unique polarization signature of magnetic realignment is a distortion of position angles of the scattered emission lines from the centrosymmetric pattern expected for pure reflection polarization . yl07 and
yl08 show that ions need at least three fine _ or hyperfine _ states in the ground level ( @xmath8 ) for a nonzero magnetic field effect .
this adds the resonance lines of nai , ki , and alii to the roster .
also , transitions that do not return to the ground state ( fluorescence ) exhibit the effect , thus opening up numerous targets in the visible and nir .
the spectral resolution required is more modest : for sensitivity against scattered dust continuum @xmath9 is adequate .
thus in principle , this effect can be observed with a ground - based moderate - resolution spectropolarimeter .
this paper describes the beginnings of such an investigation , targeted at fluorescence lines in the visible .
visible - wavelength fluorescence of oi , ni , and siii has been observed in hii regions and planetary nebulae @xcite . however , these lines are faint compared to the recombination and collision - excited lines in these regions , and the existence of alternate excitation mechanisms confuses their use .
a better environment for the exploration of these lines is the neutral medium , for instance the gas within reflection nebulae , where photoexcitation is the sole excitation mechanism .
a rough calculation suggests that the lines should be visible above the dust - scattered continuum with an equivalent width of @xmath10 , so should be visible with moderate resolution .
no such observations have been published : we here report a pilot spectroscopic investigation of the bright rn ngc2023 .
observations were obtained on 25 - 27 oct , 2006 as a part of `` performance verification '' on the new robert stobie spectrograph @xcite on the southern african large telescope , a new 10m - class telescope ( buckley , swart , & meiring 2006 ) . a north - south @xmath11 slit was offset @xmath12 east of the central star , hd37903 ( b1.5v ) .
three different grating configurations were used , covering 3800 - 4560   ( @xmath13 ) , 5425 - 6345   ( @xmath14 ) , and 7375 - 8490   ( @xmath15 ) in first order .
overall , 38 significant lines are seen : 7 unidentified , 4 primary fluorescence ( the first photon emitted after excitation , the most useful for polarimetric purposes ) , 10 secondary , and 17 unclassified ( feii , and possibly crii and tiii ) .
table 1 lists observed equivalent widths of the primary fluorescence lines , summed along the slit .
figure 2a shows the near ir observation .
the line signal goes to zero @xmath16 from the illuminating star , almost entirely interior to the well studied @xmath17 emission in the photodissociation region @xcite .
these lines provide a new velocity , temperature , and magnetic field probe of this warm neutral material .
cccccccccc ion & @xmath18 ( air ) & ew & pr / ob & grnd & upper & final & @xmath19 & @xmath20 + & @xmath21 & @xmath21 & & & & & @xmath22 & @xmath22 + siii & 5957.56 & 0.095 & 1.19 & @xmath23 & @xmath24 & @xmath23 & 50 & 0 + siii & 5978.93 & 0.184 & 1.25 & @xmath23 & @xmath24 & @xmath25 & 0 & 0 + oi & 6046.44 & 0.110 & 0.02 & @xmath26 & @xmath27 & @xmath28 & 0.00 & 0.00 + oi & 7995.07 & 0.098 & 1.82 & @xmath26 & @xmath29 & @xmath26 & 24 & 7.57 + mgii & 8115.23 & @xmath30 & 9.15 & @xmath24 & @xmath25 & @xmath31 & 10 & 0 + mgii & 9218.25 & ( 73.67 ) & ( pred ) & @xmath24 & @xmath25 & @xmath24 & 50 & 0 + alii & 8640.70 & ( 0.099 ) & ( pred ) & @xmath32 & @xmath33 & @xmath32 & 22.2 & 1.04 +    a simple model has been constructed to verify that the observed lines are seen at approximately the expected strengths , and to predict other lines that might be seen in other parts of the spectrum .
there are 11 candidate ions for fluorescence , if we require a neutral medium ( principle ion with ip @xmath34 ev ) , moderate abundance ( abundance / h @xmath35 ) , pumpable ( first resonance @xmath36 ev ) , and permitted visible - wavelength emission : ni , oi , mgii , alii , siii , ari , tiii , crii , mnii , feii , and niii .
we observe ni , oi , siii , feii , and possibly tiii and crii .
the observed equivalent widths are adequately predicted by a simple shell illuminated by a b1.5v star , @xmath37 mag , @xmath38 km / s , and no h@xmath39 absorption : the ratio of the predicted to observed equivalent widths are given in table 1 and shown in figure 2b .
all the predicted lines for our wavelength coverage are seen to within a factor of two of prediction , except for oi @xmath186048 , which may in fact be feii , and for the mgii lines @xmath408115/8120 , which are not seen , perhaps due to additional depletion . outside of our wavelength coverage , we predict a detectable line of alii at 8641   and two very strong lines of mgii at 9221 and 9244 @xmath21 ( table 1 ) .
we have not yet made predictions for tiii , crii , and feii , because their non - ls coupling results in far more complex pumping .
we can now predict polarization signals from the lines seen and predicted for ngc2023 .
the polarization is the sum of the reflection polarization - the polarization expected from an unaligned ( `` thermalized '' ) ground state - plus that due to ground state alignment .
@xcite gives the reflection polarization , @xmath41 where @xmath5 is the scattering angle and @xmath19 is the `` polarizability '' , which depends only on the angular momentum @xmath42 ( or @xmath43 ) of the ground , upper and final levels ( table 1 ) .
the polarization position angle is centrosymmetric : either perpendicular or parallel to the scattering plane .
the maximum polarization is @xmath44@xmath19 at @xmath45 ; @xmath46 corresponds to thompson scattering .
the alignment polarization will be nonzero only for alignable ions , where the ground state is aligned by pumping via all the possible uv resonance transitions .
addition of the alignment to the reflection polarization changes both the degree of polarization _ and the position angle _ , depending on the scattering angle and the 3d magnetic field orientation .
this results in a position angle that is no longer centrosymmetric , a feature unique to this process ( see figure 3a ) .
a useful measure of this signal is the deviation @xmath47 from centrosymmetric , expressed as a polarization , @xmath48 .    a plot of  @xmath49(max ) _ vs _ @xmath19 ( table 1 ; figure 3b ) for each line is useful to plan the analysis . of the ions we see or predict , two are not alignable ( mgii and siii ) , and 5 are ( oi , alii , tiii , crii , and feii ) . to deduce the magnetic field geometry at any position in a nebula , we need first to estimate the scattering angle @xmath5 . for this
we use lines of nonalignable ions with large @xmath19 , like mg ii @xmath189218 ( @xmath50 ) with equation ( 1 ) to derive @xmath5 .
lines with zero polarizability ( mgii @xmath189244 , si ii @xmath185979 ) are used to measure and subtract foreground interstellar polarization .
finally , to derive the magnetic field map , we use the map of @xmath51 for lines of alignable ions like oi @xmath187995 and alii @xmath188641 .
( if there is significant variation in @xmath5 along the line of sight , a more complex model will be required ) .
a important consideration is the role of optical depth of the uv pumping lines .
if the pumping line is `` trapped '' ( return is much more probable to the ground state than to an excited state ) , then when @xmath52 the excitation will be dominated by diffuse trapped light , which is no longer anisotropic ; any fluorescence from this level will be unpolarized , and the ground state can not become aligned .
on one hand , most fluorescent exciters are never trapped , because the fluorescent transition itself has sufficiently high probability that the exciting radiation is rapidly destroyed : the maximum number of scatterings @xmath53(escape prob ) @xcite .
( in the simple model for ngc2023 above , the fluorescence intensity is modeled as being proportional to @xmath54 or @xmath55 , whichever is smaller ) .
thus the reflection polarization of fluorescent lines should not usually be depolarized by a large factor . on the other hand , many alignable ions have lower lying trapped resonance lines that dominate the pumping of the ground state , whose effect is then to depolarize the _ alignment _ polarization .
remedies for this situation are to look at thinner nebulae , and to look at ions which do not have trapped resonance lines , such as feii .
we plan spectroscopy of ngc2023 and other nebulae to look for the predicted mgii and alii lines , followed by slit spectropolarimetry to find the best lines .
the new fabry - perot polarimetry mode of the salt rss spectropolarimeter will then be used for construction of a magnetic field map .
modeling improvements will include an feii realignment calculation and fluorescence model .
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yan and lazarian have proposed a new technique through which the magnetic field geometry in the diffuse interstellar medium , or in circumstellar matter , could be determined from the linear polarization of interstellar absorption or fluorescence emission lines from ions pumped by an anisotropic illuminating flux . 
 new long - slit spectroscopic observations of the reflection nebula ngc2023 , obtained with the southern african large telescope robert stobie spectrograph ( rss ) , have detected a number of atomic fluorescence lines of oi , ni , siii , and feii for the first time in a neutral medium . a model which predicts these lines and others illustrates 
 which lines would be appropriate targets for an rss spectropolarimetric investigation of this new diagnostic .