Unveiling Vela - Time Variability of Interstellar Lines in the Direction of the Vela Supernova Remnant II. Na D and Ca II
N. Kameswara Rao, David L. Lambert, Arumalla B. S. Reddy, Ranjan Gupta, S. Muneer, Harinder P. Singh
aa r X i v : . [ a s t r o - ph . S R ] J a n Mon. Not. R. Astron. Soc. , 1– ?? (2014) Printed 27 July 2018 (MN L A TEX style file v2.2)
Unveiling Vela - Time Variability of Interstellar Lines inthe Direction of the Vela Supernova Remnant II. Na Dand Ca II ⋆ N. Kameswara Rao , , David L. Lambert , Arumalla B. S. Reddy , Ranjan Gupta ,S. Muneer & Harinder P. Singh † , Indian Institute of Astrophysics, Bangalore 560034, India The W.J. McDonald Observatory and Department of Astronomy, The University of Texas, Austin, TX 78712-1083, USA Inter University Centre for Astronomy and Astrophysics, IUCAA Pune, 411007 India Department of Physics & Astrophysics, University of Delhi, Delhi 110007 India
27 July 2018
ABSTRACT
In a survey conducted between 2011-12 of interstellar Na i D line profiles in the direc-tion of the Vela supernova remnant, a few lines of sight showed dramatic changes inlow velocity absorption components with respect to profiles from 1993-1994 reportedby Cha & Sembach. Three stars - HD 63578, HD 68217 and HD 76161 showed largedecrease in strength over the 1993-2012 interval. HD 68217 and HD 76161 are as-sociated with the Vela SNR whereas HD 63578 is associated with γ Velorum windbubble. Here, we present high spectral resolution observations of Ca ii K lines obtainedwith the Southern African Large Telescope (SALT) towards these three stars alongwith simultaneous observations of Na i D lines. These new spectra confirm that theNa D interstellar absorption weakened drastically between 1993-1994 and 2011-2012but show for the first time that the Ca II K line is unchanged between 1993-1994and 2015. This remarkable contrast between the behaviour of Na D and Ca II K lineabsorption lines is a puzzle concerning gas presumably affected by the outflow fromthe SNR and the wind from γ Velorum.
Key words:
Star: individual: ISM: variable ISM lines: Supernova Remnants :other
Vela supernova remnant (SNR) is the closest relic of of astellar explosion to earth, located at a distance of 287 ± ⋆ Based on observations obtained with Southern African LargeTelescope (SALT) and The Vainu Bappu Telescope (VBT). † E-mail: [email protected] (NKR); [email protected](DLL);[email protected] (SM); [email protected] (RG);[email protected] (HPS) profiles of Ca ii K and Na i D lines superimposed on stellarspectra.Recently, we completed a study of high resolution Na i D line profiles towards sixty four OB stars in the direction ofVela SNR , mainly carried out during 2011-2012, and com-pared them with Ca ii K and Na i D line profiles of the samestars earlier obtained by Cha & Sembach (2000) in the pe-riod 1993-1996 at comparable spectral resolution (Rao et al2015 -hereafter Paper I). Comparison of the profiles of Na i Dfrom these two epochs revealed major decreases in low veloc-ity absorption components towards a few stars. When PaperI was written, we did not have Ca ii K profiles for compar-ison with K line observations of Cha & Sembach (2000) toconstrain the cause of this Na D variability. Subsequently,we could obtain with the high-resolution spectrograph atSouthern African Large Telescope (SALT) Ca ii K profilesfor three of the stars exhibiting dramatic weakening of NaD absorption. In the present paper, we report on the Ca ii K c (cid:13) N. K. Rao et al. and Na i D lines towards three stars in which D line absorp-tion at low velocities weakened greatly between 1993-1994and 2011-2012. The surprising result is that in sharp con-trast to the Na D lines the Ca ii K line in 2015 has the sameprofile in all three cases as it did in 1993-1994.Earlier studies of interstellar absorption lines towardsvarious stars in and around the Vela SNR indicated thathigh velocity (i.e >
100 km s − ) components, most com-monly seen in Ca ii profiles, manifest shocked gas associatedwith the SNR (Wallerstein, Silk & Jenkins 1980; Jenkins,Wallerstein & Silk 1984). Sushch, Hnatyk & Neronov (2011)pointed out that only stars with distances greater than 500pc show these high-velocity components in their spectrum(see Paper I figure 18, Cha, Sembach & Danks 1999). Forreference, the pulsar is at about 290 pc and the three starsof interest here are at 300-500 pc and do not show high-velocity interstellar components. The diameter of Vela su-pernova remnant is estimated to be about 8.3 degrees (As-chenbach 1993; Aschenbach, Egger & Trumper 1995). Twoof the three stars discussed here, HD 68217 and HD 76161,are located in the outer edge of the ROSAT X-ray image ofthe SNR. The third one, HD 63758, is located in the γ Velwind bubble region.
Our high-resolution spectra of the three stars discussed herewere obtained at two observatories. The resolving power R = λ/dλ of the spectra is comparable to that of the baselinespectra by Cha & Sembach (2000) who reported a value of R = λ/dλ ≃ R = λ/dλ , the spectral resolving power with a60 µ m m slit, was 72000. The spectrum covers the wave-length range of 4000 to 10000˚A with gaps. Beyond about5600˚A the echelle orders were incompletely recorded on aE2V 2048 × ii H & K lines occur in the insensitive region.The wavelength calibration was done using Th-Ar hollowcathode lamp exposures that were obtained soon after theexposures on the star.New high resolution spectra of Ca ii H & K lines alongwith Na i D lines have been acquired with the high resolutionspectrograph (HRS) on the Southern African Large Tele-scope (SALT) on 2015 December 15 (Bramall et al. 2010).The HRS blue spectrum with a 1.6 arc sec fiber provides aresolving power of 66700. Spectra with Both blue arm (3674-5490 ˚A) and red arm (5490-8810 ˚A) have been obtained. TheS/N ratio of the red spectrum in the region of the Na D linesis comparable to that of the VBT spectra ∼ − ii K line are of a lowerS/N ratio ( ∼ −
50) than the spectra provided by Cha &Sembach (2000). The resolving power of the SALT spectrais 63000 at the D lines as measured from the width of weaktelluric lines and comparable at K line.IRAF routines have been used for spectral reductionsnamely flat field corrections, bias subtractions, wavelength calibration and corrections for telluric line blending. Weadopted the same local standard of rest (LSR) of Cha &Sembach (2000) for converting heliocentric velocities to LSRvelocities. Decomposition of various components from theobserved line profiles are accomplished by fitting Gaussianprofiles. Central radial velocity V LSR , full width at half max-imum and equivalent width of the components have thusbeen obtained from these fits. The values of these parame-ters listed in Cha & Sembach (2000) were used as startingvalues for the gaussian fits and further changes were madeso as to make both D and D profiles yield the same num-ber of components, same V LSR and also similar full widthat half maximum. The combined gaussian fits are made tomatch the observed profiles such that no residuals are leftover the noise in the surrounding continuum. These fits areof doubtful validity when the lines are saturated, as is thecase for some Na D profiles. When multiple observations areavailable an average equivalent width of the components ispresented in tables. The SALT spectra in the D line regionare not corrected for telluric line contamination, since si-multaneous telluric line standard star observations were notobtained. As a result radial velocity and equivalent width offew D components are affected by telluric lines. However, D profiles are fine. The LSR radial velocities of the Na i Dline components obtained from SALT profiles agree with in ± − with the ones measured from VBT profiles. I COLUMN DENSITYHAVE BEEN OBSERVED
Low V LSR velocity clouds that are recognised through Na Dand Ca ii absorption components are usually considered tobe associated with diffuse interstellar medium in the spiralarms. However interstellar clouds in the proximity of theVela supernova remnant might have experienced interactionswith the remnant through radiation or collisions and thus,their physical conditions and kinematics altered.Our survey of interstellar Na D lines (Paper I) showedthat the most of the stars observed with VBT during 2011-12had their Na D profiles unchanged compared to 1993-1996observations of Cha & Sembach (2000). Most of the low ve-locity components, generally in the V LSR range of ±
25 to 0km s − (Cha, Sembach & Danks 1999) of Na D absorptionmight have their origins in clouds that occur in the interven-ing spiral arms. A remarkable result of Paper I was a greatdecrease in strength (almost disappearance) of low-velocityabsorption components in Na D lines in just three stars –HD 63578, HD 68217 and HD 76161, in the period between1993 − − ii K line pro-files differ from those that were obtained by Cha & Sembachduring 1993-1994, in a similar way to the large changes seenin the Na D profiles between 1993-1994 and 2011-2012? and(ii) Have the Na D profiles evolved further in the intervalthree-four year between the VBT and SALT observations?(Cha & Sembach noticed changes in strength and velocityin some high velocity absorption components in their ob-serving period of about three years, but our VBT spectra c (cid:13) , 1– ?? nveiling Vela-SALT Table 1.
Absorption Lines of Ca ii K & Na i towards HD 63578 Ca ii K Na i C & S a Na i (VBT) b Na i (SALT) c C&S SALT D D D D D D V LSR
Eq.w Eq.w V LSR
Eq.w Eq.w V LSR
Eq.w Eq.w Eq.w Eq.wkm s − (mA) (mA) km s − (mA) (mA) km s − (mA) (mA) (mA) (mA) −
14 24 28 −
13 31 15 (27) 14 − − ∗
216 34 43 6 232 191 230 19111 325 270 22 5 a C&S: Cha & Sembach (2000) observations from 1993 b VBT: Average of four nights from 2011- 2012 (see Paper I) c SALT: Observation from 2015 December 15 ∗ : Affected by telluric line Table 2.
Absorption Lines of Ca ii K & Na i towards HD 68217. Ca ii K Na i (C & S) a Na i (VBT) b Na i (SALT) c C&S SALT D D D D D D V LSR
Eq.w Eq.w V LSR
Eq.w Eq.w V LSR
Eq.w Eq.w Eq.w Eq.wkm s − (mA) (mA) km s − (mA) (mA) km s − (mA) (mA) (mA) (mA) −
18 17 −
19 9 5 ∗ − . − − ∗
32 43 12 2 41 22 41 179.0 20 25 9 20 16 9 21 10 21 9 a C&S: Cha & Sembach (2000) observations from 1994 b VBT: Average of four nights from 2011- 2012 (see Paper I) c SALT: Observation from 2015 December 15 ∗ Affected by telluric line
Table 3.
Absorption Lines of Ca ii K and Na i towards HD 76161 Ca ii K Na i (C & S) a Na i (VBT) b Na i (SALT) c C&S SALT D D D D D D V LSR
Eq.w Eq.w V LSR
Eq.w Eq.w V LSR
Eq.w Eq.w Eq.w Eq.wkm s − (mA) (mA) km s − (mA) (mA) km s − (mA) (mA) (mA) (mA) − − − ∗
42 38 43 2 60 32 63 289 395 365 13 8 4 10 627 61 32 a C&S: Cha & Sembach (2000) observations from 1993 b VBT: Average of four nights from 2011- 2012 (see Paper I) c SALT: Observation from 2015 December 15 ∗ Affected by telluric linec (cid:13) , 1– ?? N. K. Rao et al.
LSR
V (km s ) −1 N o r m a li z ed I n t en s i t y −100 0 1000.00.10.20.30.40.50.60.70.80.91.01.1−200 −100 0 100 2000.00.10.20.30.40.50.60.70.80.91.01.1 Figure 1. (Left): Profile of Na i D in HD 63578 obtained in 2011 (red line) shown superposed on the profile of Na i D observed in1993 by Cha & Sembach (2000)(black line). The blue-shifted absorption components in the 1993 spectrum (black line) is absent in the2011 spectrum. (Right): Ca ii K profiles of HD 63578 obtained with SALT in 2015 (red line) overplotted on profile obtained by Cha &Sembach (2000) in 1993. -50 0 50 10000.51
Figure 2.
The profiles of Na i D in HD 63578 obtained during 2012 (blue line) are superposed with profiles obtained in 2015 withthe SALT (red line). The SALT spectrum was not corrected for (weak) telluric lines. The dotted lines show the velocities of the fourGaussian components fitted to the profiles – see Table 1. have a limited information on such absorption components.In particular, the three stars observed at SALT have notshown high velocity components in any available spectra.High-velocity components appeared more frequently in theCa ii K line than the Na i lines.) HD 63578:
For this star located at a distance of 150 pcbehind γ Velorum and seemingly in the bubble generatedby γ Velorum’s wind ( se Paper I) , Figure 1(left) showsthe much weaker low velocity Na D line in 2011 relative to that in 1993. The figure also shows that the D profile didnot change further between 2011 and 2015 (Figure 2) . Instriking contrast, the Ca K profiles are unchanged between1993 and 2015 (Figure 1-right).Gaussian components extracted from the spectra aresummarized in Table 1. These show that the Ca K line’stwo components are unchanged in velocity and equivalentwidth between 1993 and 2015 except perhaps for a possibleincrease in the redder component in the SALT spectrum. c (cid:13) , 1– ?? nveiling Vela-SALT N o r m a li z ed I n t en s i t y V (km s )
LSR −1 −200 −100 0 100 2000.40.50.60.70.80.91.01.1−200 −100 0 100 2000.40.50.60.70.80.91.01.1
Figure 3. (Left panel):Profile of Na i D in the sight line to HD 68217 as obtained in 1994 by Cha & Sembach (black line) superposedon the D profile obtained on 2012 January 16 with the VBT (red line). The strong absorption component present at V LSR = − − in 1994 is conspicuously weakened by 2012. (Right panel): The Ca ii K profile obtained with SALT in 2015 of HD 68217(red line)is overplotted on the profile obtained in 1994 by Cha & Sembach (2000) (black line). There may be a slight weakening of the sharpabsorption component at − − by the time the SALT profile was obtained. -50 0 50 1000.20.40.60.811.21.4 Figure 4.
Profiles of Na i D and D obtained with VBT in 2011 (blue lines) are compared with profiles obtained on 2015 December 25with SALT (red line). The profiles are very similar indicating no significant change occurred between 2011 and 2015. The SALT spectrumwas not corrected for (weak) telluric lines. The dotted lines show the velocities of the Gaussian components fitted to the Na D profiles –see Table 2. Cha & Sembach note that the two velocity components aretraceable back to 1971 - 1977.At Na D, Cha & Sembach fitted their 1993 spectrumwith strong components (Table 1) at − − which differ by several km s − from their velocities of − − obtained from the much weaker Ca K line.For the VBT and SALT Na D lines, there are components at −
13 and +6 km s − apparently coincident with the Ca Kcomponents plus another weak Na D component at − − . Jenkins, Wallerstein & Silk (1984) obtained an ultra-violet spectrum of the star with the IUE on 1979 June 15.They measured for both C i , S ii , and Si ii –Fe ii lines LSRradial velocities of +9, +7 and +2 km s − , respectively. The c (cid:13) , 1– ?? N. K. Rao et al.
LSR
V (km s ) −1 N o r m a li z ed I n t en s i t y −200 −100 0 100 2000.00.10.20.30.40.50.60.70.80.91.01.1−200 −100 0 100 2000.00.10.20.30.40.50.60.70.80.91.01.1 Figure 5. (Left panel): Profiles of Na i D in the sight line towards HD 76161 obtained with VBT on 2011 December 25 (red line) issuperposed on the 1993 profile obtained by Cha & Sembach (2000) (black line). Strong absorption components at V LSR = 9 and 27 kms − have almost disappeared by the time of VBT observations in 2011. (Right panel): Ca ii K profiles of HD 76161 obtained with SALTin 2015 (red line) overplotted on 1993 profile obtained earlier by Cha & Sembach (2000). -50 0 50 1000.20.40.60.811.21.4
Figure 6. Na i D and D profiles obtained in 2011 with (blue lines)VBT are compared with profiles obtained on 2015 Dec 15 withSALT (red line). The profiles are very similar indicating no significant change occurred between 2012 and 2015. The SALT spectrum wasnot corrected for telluric lines. The dotted lines show the velocities of the Gaussian components fitted to the Na D lines – see Table 3. +9 km s − velocity is coincident with the +9 km s − com-ponent of Ca K. The − − component that has beenpresent in 1993 spectrum in Na D lines was not seen in ei-ther C i or in singly ionized lines of Ca, S , Si and Fe. Thus,this component probably refers to a cold gas cloud seen onlyin Na D. HD 68217:
This star lies near the edge of the ROSATimage of the Vela SNR. Figure 3(left) shows that a sharp NaD component at − − almost vanished by 2011-2012 but remained without additional change in 2015 (Figure 4).Table 2 shows that the Gaussian components for the Ca Kline have close counterparts in the Na D profiles and, inparticular, the sharp feature at − − which weakenedgreatly at Na D may also have weakened at Ca K (Figure3-right). The four Na D components identified by Cha &Sembach are present in the VBT and SALT spectra withthe − − feature weaker in the latter spectra. A NaD spectrum obtained in 1989 by Franco (2012) with the c (cid:13) , 1– ?? nveiling Vela-SALT same instrument used by Cha & Sembach shows the − − feature weaker than it was in 1994 but stronger than inVBT and SALT spectra which suggests that the feature’sevolution is not a simple one. HD 76161:
This star which like HD 68217 lies nearthe edge of the ROSAT image of the Vela SNR (paper I)provides the third and the extreme example in which lowvelocity strong Na D absorptions present in 1993 had almostdisappeared by 2011 - see Figure 5(left) : the saturated ab-sorption component at V LSR +9km s − has vanished almostcompletely by 2011. A redward weaker absorption compo-nent at +27 km s − present in 1993 also weakened dras-tically by 2012. However a weak absorption component atabout − − seems unchanged over the period of 1993to 2012 baseline. The Na D profile remained unchanged inthe period 2011 to 2015 (Figure 6).The 1993 Ca ii K profiles of 1993 and 2015 (Figure 5-right) appear with components at − − (Table3). The Ca ii K profile obtained between 1971-77 period byWallerstein, Silk & Jenkins (1980) appears to be similar tothe profile observed by Cha & Sembach (2000) in 1993.Jenkins, Wallerstein & Silk (1984) measured LSR ra-dial velocity of S ii and Si ii + Fe ii as − − and − − respectively from their IUE spectrum obtained in1979 September 26, very similar values to that of Ca ii Kcomponents measured bu Cha & Sembach (2000) in 1993.Later, Nichols & Slavin (2004) detected C i lines at theLSR velocity of 8 km s − in the same IUE spectrum ob-tained in 1979 . This velocity corresponds to the strong Na i component that disappeared after 1993. It appears that astrong neutral cloud was present at +9km s − for at leasta period of 14 years prior to 1993 and then vanished sometime in the next 18 years.Nichols & Slavin (2004) investigated C i lines in 54 starsin Vela SNR region including HD 76161, and noted the red-shift of C i lines with respect to ionized lines and suggestedthat C i lines from the ground-state of the atom are a combi-nation of two absorption components: one near zero velocitywhich might have its origins in the clouds in the spiral armalong the sight lines to the star and the other component at ahigher velocity arising from cloud that might have interactedwith the Vela SNR. They also suggest that redshift observedin C i ground-state lines might have its origins in a H i shellon the back side of the remnant. Dubner et al. (1998) discov-ered a shell of H i through 21 cm emission around Vela SNRthat follows closely the ROSAT’s outer X-ray bright shell.This 30 km s − expanding The H i shell is thought to be aresult of recombined postshocked gas behind the advancingshock front. There is the possibility that the cold neutralcloud that disappeared in the direction of HD 76161 mighthave been a shocked cloud. The focus of the paper has been three stars in the directionof the Vela SNR in which a severe weakening of low-velocityabsorption components of interstellar Na D lines occurredbetween 1993-1994 and 2011-2012 (Rao et al. 2016). Suchlow-velocity components are assumed to belong to the localinterstellar medium and not to be an immediate product ofthe Vela SNR or other energetic sources in the vicinity of the Vela nebula. A few earlier reports for stars behind theVela SNR have noted modest variations in the Na D linesat low velocity - see, for example, observations of the visualbinary HD 72127 (Hobbs, Wallerstein & Hu 1982; Hobbset al. 1991; Welty. Simon & Hobbs 2008). The magnitudeof the weakening reported by us earlier is unprecedented inpublished studies of interstellar Na D lines (see, for exam-ple, McEvoy et al. 2015) and is plausibly attributable to in-teractions between ambient diffuse interstellar gas and thehigh-velocity gas in and around the Vela nebula from theVela supernova and the winds from massive stars. It seemshighly pertinent to note that out of forty one stars with NaD lines observed previously by Cha & Sembach and againat the VBT none showed a comparable strengthening of theNa D lines between 1993–1994 and 2011–2012. (As noted forHD 68217, A low-velocity component Na D did strengthenbetween 1998 and 1994. Also, HD 73882, a star behind theVela SNR showed a strengthening of the Ca i | V LSR ∼ | kms − ) absorption components seen in stars with distancesgreater than about 500 pc. High-velocity gas was first re-ported by Wallerstein & Silk (1971) and Thackeray & War-ren (1972) from observations of the Ca ii K line with exten-sive follow-up at the Ca ii K line and Na D lines by Cha &Sembach (2000). (High-velocity Na D gas associated withthe SNR Monoceros Loop has been shown to be variable(Dirks & Meyer 2016).) High-velocity gas associated withVela has been studied in the ultraviolet (Jenkins, Waller-stein & Silk 1976; Wallerstein, Silk & Jenkins 1980; Jenkinset al. 1981; Jenkins, Wallerstein & Silk 1984). With respectto the Na D and Ca K line, a characteristic of the high-velocity gas is the several 100-fold increase with respect tothe value for low velocity gas of the column density ratio ofCa ii to Na i which is attributed to the destruction of inter-stellar grains and release of substantial amounts of Ca (andother elements) but not Na which is little depleted by grains(Spitzer 1978; Danks & Sembach 1995; Sembach & Danks1994).Surely, the outstanding characteristic of the low-velocity interstellar lines of the three sightlines where theNa D lines are strikingly weakened between 1993-1994 and2012-2015 is that the Ca K line shows no detectable changebetween 1993-1994 and 2015 in two cases (HD 63578 andHD 76161) and a possible mild weakening in the third case(HD 68217) where the central depth of the component coin-cident in velocity with the varying Na D feature drops from77% in 1994 to 88% in 2015, a drop only slightly larger thanthe the amplitude of the noise in the SALT spectrum. Inshort, the Ca K line is effectively unaffected by the factorsresponsible for the large Na D changes and, in particular,the Ca K’s velocity component coincident with the variableNa D component are unaffected. This contrast between NaD and Ca K constrains the factors driving the weakening ofthe Na D line.Estimation of the Na i column density for two of thethree examples is an uncertain procedure for the 1993-1994spectra because the dominant components are saturated.For the VBT and SALT spectra, the variable component of c (cid:13) , 1– ?? N. K. Rao et al. the Na D lines is little saturated; the equivalent widths ofthe D and D lines approach the 2:1 weak line limit. For HD68217, the − − component appears to provide boththe Na D and Ca K line and assuming that the lines areunsaturated, the column density ratio N(Ca ii )/N(Na i ) wasabout 3 in 1994 and increased to 17 in 2015. For the othertwo stars, the ratio N(Ca ii )/N(Na i ) for the highly-variablecomponent was between about two and four in 2015. The1993-1994 ratios were obviously much smaller because of thelarger Na i column densities and these ratios appear typicalof diffuse clouds (Danks & Sembach 1995). The ratio of 17for HD 68217 in 2017 is typical of values for high-velocityclouds in the Vela SNR (Sembach & Danks 1994) but thechange between 1993-1994 and 2015 seems unlikely to bedue to release of Ca from grains because the Ca K line’sequivalent width is unchanged over this interval and the in-crease in the ratio arises from loss of neutral Na presumablyto ionization. High-velocity clouds moving tangentially tothe line of sight may appear as low velocity clouds and thislikelihood is increased for stars such as our trio located nearthe edge of the SNR.Interactions between the SNR and an ambient diffusediffuse cloud may break the cloud into smaller parts. Pakho-mov, Chugal & Iyudon (2012 – see also Klein, McKee & Col-lela 1994) discuss how a fast-moving shock interacts witha diffuse cloud. Three stages of interaction are envisaged.First, the shock propagates through the cloud. In the sec-ond stage, the cloud is accelerated. Finally, the cloud is frag-mented. It seems that the interaction can destroy a small(diameter of a few au) cloud in a few years. Although de-tailed calculations remain to be done to show that almostcomplete disappearance of Na D lines may be achieved withalmost no change of the Ca K line, it seems, as the refereehas pointed out, unlikely that large-scale fragmentation canlead to a large Na D reduction without a change in Ca Kand, moreover, changes in radial velocity are likely to ac-company column density changes.Nonetheless, the fact that these remarkable changes inNa D lines unaccompanied by Ca K line changes occur onsight lines to SNR or through a region crossed by a vigorousstellar wind and have not been seen sight lines through theambient diffuse stellar medium suggests that shocks may bethe key to understanding our remarkable discovery. Shockfronts even at low velocities can emit Ly-alpha photons(Shull & McKee 1979). Lyman α at 10.2 eV can ionize neu-tral Na with its ionization potential of 5.14 eV but not Ca + with its ionization potential of 11.9 eV. As the front ap-proaches the cloud, the increase in Lyman α flux could berather abrupt; the zone containing Lyman α photons aheadof the front can be narrow because Lyman α photons scattermany times in a neutral hydrogen medium. (This appealingargument we owe to the referee.)A strictly geometrical explanation but perhaps over-looking the SNR and stellar wind association may be pos-sible. The column density ratio of Na D/Ca K for diffuseclouds spans the range from 300 to 0.02 (Welty, Morton &Hobbs 1996). If a thin cloud of high Na D/Ca K moved outof the line of sight to our three stars, the Na D line wouldweaken sharply but, perhaps, the Ca K line would be largelyunchanged. Observations of sight lines through the generalinterstellar medium do not support this idea as a commonan event as we find towards the Vela SNR. The principal next challenge suggested by our VBT andSALT spectroscopy of Na D and Ca K lines is to catchand follow large scale changes occurring along a line ofsight. Panoramic optical spectroscopy will be able to ob-serve the behaviour of many interstellar lines (see, for ex-ample, Pakhomov, Chugai & Iyudin 2012). Since there is asyet no predictor of which star is about to undergo a changein its interstellar lines, detection of an onset will requireroutine high-resolution monitoring of a set of stars in andaround the Vela SNR but this presents a severe schedulingchallenge and involves a considerable consumption of observ-ing time. Although observation of transient phenomena is a‘hot’ contemporary topic, its remit does not yet to extendto exploration of the questions raised by our paper. We thank SALT astronomer, Brent Miszalski, for his con-siderable help in conducting the HRS observations for us.We also thank Dr. Ed Jenkins, the referee, for his insightfulcomments about how shocks may account for our results andother corrections. We also would like to thank Baba Vargh-ese for providing the figures of Ca ii K superpositions. Thisresearch has made use of the SIMBAD database, operated atCDS, Strasbourg, France. We also would like to thank staffof the VBO at Kavalur for their help with observations.
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