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the recent discovery of 10 hypervelocity stars ( hvss ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) has raised many questions about their nature and origin . the most widely accepted ejection mechanism , proposed by @xcite , involves the encounter of a close binary with a supermassive black hole ( smbh ) . other possible mechanisms ejecting stars from the galactic center involve intermediate - mass black holes ( imbhs ; e.g. * ? ? ? * ; * ? ? ? * ) , a binary massive black hole ( bmbh ; e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) , or a cluster of stellar mass black holes around the smbh @xcite . @xcite claimed that the rotational velocities of hvss should be lower than those measured for single stars of the same spectral type if they originated in binaries , because of tidal effects . he predicted that the rotational velocities of the known b - type hvss should be @xmath2 km  s@xmath1 , based on values compiled by @xcite for b - stars in binaries . @xcite predicted high rotational velocities for hvss that were ejected by a very close encounter with an imbh in the galactic center , however such encounters are very unlikely . these predictions can not be tested with existing observations , as the low resolution of the discovery spectra of most hvss is not sufficient to determine projected rotational velocities ( @xmath3 ) . the only hvs with high resolution spectroscopy and a @xmath3 measurement is he 04375439 , found by @xcite . it has @xmath4  km  s@xmath1 @xcite , in agreement with the prediction of @xcite . however , @xcite and @xcite measured half - solar metallicity for this early b - star , establishing its origin in the large magellanic cloud ( lmc ) . the possible ejection mechanisms for this star include an interaction with an imbh or a smbh , and a dynamical interaction of a single star in a dense cluster . this example demonstrates the importance of high resolution spectroscopy for understanding this newly discovered class of objects .    of the remaining hvss , hvs2 ( or us708 ; * ) , is classified as an evolved sdo star and reasonably well understood . however there is some ambiguity in the nature of the late b - type hvss , since at their temperatures and gravities , the blue horizontal branch ( bhb ) crosses the main sequence . hot bhb stars generally have low rotational velocities and peculiar chemical abundances @xcite , thus high resolution spectroscopy of these faint hvss can determine their nature by measuring their atmospheric parameters , chemical abundances and @xmath3 . in addition , time series photometry can reveal pulsations and confirm their main sequence nature , as was done for hvs1 by @xcite . motivated by the lack of @xmath3 and stellar parameter measurements for most of the known hvss and the possibility of testing the nature of the smbh in the center of our galaxy , we performed high resolution spectroscopy of two hvss . in this letter we present our results . we collected spectra of hvs7 and hvs8 ( sdss j113312.12@xmath5010824.9 and j094214.04@xmath5200322.1 ) with the blue chip of the mike spectrograph @xcite installed at the 6.5-m magellan clay telescope at las campanas observatory ( chile ) , on two half nights on ut 2008 january 1819 . each star was observed twice , with individual exposure times between 900 and @xmath6 , using a @xmath7 slit and 3@xmath83 binning . the total exposure times were @xmath9 for hvs7 and @xmath10 for hvs8 . the resolution of the spectra is r = 32,000 at 4500  . the spectra were extracted using the mike reduction pipeline @xcite . the extracted spectra for each star were then averaged , normalized and merged . the wavelength coverage of the merged spectra is 3900 - 5050  , with an average s / n - ratio per pixel of 15 for hvs7 and 14 for hvs8 , based on the extracted continuum around 4500  . these s / n - ratios and our spectral resolution are sufficient to distinguish between high ( @xmath11 km  s@xmath1 ; * ? ? ? * ) and low ( @xmath12 km  s@xmath1 ; * ? ? ? * ) @xmath3 values for b - stars . next , we corrected the wavelength scale for doppler shift , to allow comparison of the spectra with models ( see  3 ) . we measured the heliocentric radial velocity of each star using the iraf cross - correlation package rvsao @xcite and the grid of models described in  3 . table  [ tab : rv ] lists our results and the values previously reported by @xcite . 4 discusses the implications of our new radial velocity measurements . our high resolution spectra allow direct determination of the effective temperature @xmath13 , surface gravity @xmath14 , and @xmath3 of the stars by comparing synthetic model spectra to the observations . the s / n - ratio of the data is however too low to reliably measure abundances . we generated a grid of synthetic spectra using the lte atlas9 models and opacities developed by @xcite . the grid covers @xmath13 between 800015000  k in steps of 1000  k , and @xmath14 between 3.05.0 in steps of 0.25 dex . the metallicity was set to solar , assuming that the hvss are ejected from the galactic center , where abundances are solar or supersolar @xcite . for the macro- and micro - turbulence velocities we adopted 0 and 2 km  s@xmath1 , which are typical for late b - stars @xcite . the models were broadened by 0.15    to match mike s instrumental profile and resampled to a dispersion of 0.03  / pix to match the dispersion of the stellar spectra . finally , we convolved each model with rotational profiles between 10350 km  s@xmath1 in 10 km  s@xmath1 velocity increments . simultaneous fits to @xmath13 , @xmath15 and @xmath3 were performed for each star by iteratively comparing each model to the data . the agreement between each model and the observed spectra is quantified by the spectroscopic quality - of - fit parameter , @xmath16 ( normalized @xmath17 ) , defined by @xcite and given by the equation @xmath18    where @xmath19 is the number of points in the spectrum , and @xmath20 and @xmath21 are the root mean squared deviation between each model and the stellar spectrum , and the smallest value of the rms found . @xmath16 = 0 gives the best model fit , and @xmath16 = 1 defines the statistical 1@xmath22 confidence interval of the result . the following subsections describe the derivation of @xmath13 , @xmath15 and @xmath3 for each target . the spectrum of hvs7 ( v=17.80 mag , s / n=15 ) includes four balmer lines ( @xmath23  @xmath24 ) from which @xmath13 , @xmath14 and @xmath3 can be estimated . we also detect , , and lines that can in principle be used to further constrain @xmath13 and @xmath3 , however the two main @xmath13 indicators ( and ) have anomalous line strengths and can not be used to constrain the @xmath13 . we are therefore left with only the balmer lines that are simultaneously sensitive to @xmath13 , @xmath14 and @xmath3 , but can still provide non - degenerate values of these parameters for late b - type stars ( see  4 of * ? ? ? we performed two tests to verify that balmer lines alone are sufficient to simultaneously derive the three parameters : a ) we applied our analysis to synthetic spectra with added random noise matching the s / n - ratios of the observations , and in all cases recovered the input values within errors , b ) we applied our analysis to a high s / n spectrum of a late b - type star ( hr7447 , b5 iii ) , kindly provided by l. lyubimkov . our analysis yielded @xmath13 = 14,000 @xmath0 1000 k , @xmath14 = 3.75 @xmath0 0.25 dex , and @xmath3 = 70 @xmath0 20 km  s@xmath1 , in agreement with the parameters derived by ( * ? ? ? * ; * ? ? ? * @xmath13 = 13,400 k , @xmath14 = 3.64 dex , @xmath3 = 76 km  s@xmath1 ) .    we then proceeded to fit the spectrum of hvs7 for @xmath13 , @xmath14 and @xmath3 . we performed several tests to determine the stability of the best fit solutions . we ran fits to the entire spectrum , 100     windows centered on each balmer line ( to ensure that the wings and some continuum are included ) , portions of the spectrum outside the balmer lines , and 1020     windows centered on metal lines . in the last two cases we had to fix @xmath13 and @xmath14 to the values from the fits to the entire spectrum and the balmer lines and only fit for @xmath3 . all the tests give fully consistent results , with the following best fit parameters : @xmath13 = 12,000 @xmath0 1000 k , @xmath14 = 3.50 @xmath0 0.25 dex , and @xmath3 = 60 @xmath0 17 km  s@xmath1 . the @xmath16 minimization results for the full spectrum are shown in figure [ fig : conts ] . we have adopted conservative errors for @xmath13 and @xmath14 equal to the grid step size , versus their smaller 1@xmath22 statistical errors . the statistical 1@xmath22 errors for @xmath3 ( horizontal dotted line in the @xmath16 vs. @xmath3 plot in figure [ fig : conts ] ) , are @xmath25 and @xmath26 km  s@xmath1 , however visual comparison of the models to the observed spectrum show they are too large . instead we adopted the errors resulting from the fits to individual metal lines . the left panel in figure [ fig : hlines ] compares the best fit model to the balmer lines of hvs7 . figure [ fig : metals ] shows metal lines detected in hvs7 with @xmath3 = 40 , 60 , and 80 km  s@xmath1 models overplotted . the purpose of this plot is two - fold ; the left - side panels show how the k and @xmath274233 lines ( @xmath3 is derived from these two and the @xmath284549 and 4583 lines ) give @xmath3 = 60 km  s@xmath1 as the best model fit , and their depths agree with the solar abundance adopted in the models . the @xmath3 from these lines also agrees with the fits to the full spectrum and the balmer lines . the right - side panels show the behavior of the doublet , the and lines . none of the models in the grid can reproduce the depths of those lines . while and seem depleted in the atmosphere of hvs7 , the 4128/4130 lines seem strongly enhanced . the models can not reproduce either the depth nor the line strength ratio of the and lines . the enhancement of the 4128/4130 doublet is even more significant when taking into account that the kurucz lte atlas9 models overpredict the strengths of these lines . this problem persists even after including non - lte corrections @xcite . abundance peculiarities have been noticed before in bhb stars by several authors @xcite ; however , most of these are very slow rotators ( @xmath29 km  s@xmath1 ; * ? ? ? the spectrum of hvs8 ( v=18.09 mag , s / n = 14 ) has a s / n similar to the hvs7 spectrum , however , inspection of the spectrum of hvs8 for metal lines gives null results . this can be explained by very low metal abundances , strong depletion , or highly broadened metal lines . the @xmath3 obtained below points towards the latter case . to derive @xmath13 , @xmath14 and @xmath3 we used only the spectrum above 4000    because of problems with the continuum normalization at shorter wavelengths . as with hvs7 , we simultaneously fit for @xmath13 , @xmath14 and @xmath3 by iteratively comparing the spectrum of hvs8 to our model grid . we ran fits to the entire spectrum and 160     windows centered on the balmer lines , which gave consistent parameters : @xmath13 = 11,000 @xmath0 1000 k , @xmath14 = 3.75 @xmath0 0.25 dex , and @xmath3 = 260 @xmath0 70 km  s@xmath1 . the lack of metal lines in the spectrum of hvs8 is consistent with a high @xmath3 that results in strong line broadening . the error in @xmath3 in this case comes directly from the @xmath16 = 1 statistical 1@xmath22 result , as visual comparison of the spectrum to the models does not allow us to place a finer constraint . the right - side panel in figure [ fig : hlines ] compares the best fit model to the @xmath23 , @xmath30 and @xmath31 lines of hvs8 . the flat - bottomed cores of the balmer lines , which are the most sensitive regions to @xmath3 , clearly illustrate that hvs8 rotates faster than hvs7 . the new radial velocity observations in table [ tab : rv ] provide a third epoch for each star and allow to check for variations . our radial velocity measurement for hvs7 is identical to the values reported by @xcite and @xcite , within errors . such measurements provide clues to the nature of hvss . as pointed out by brown et al . , determining the nature of late b - type hvss is not straightforward because late - type main sequence b - stars and hot blue horizontal branch ( bhb ) stars have identical atmospheric parameters . bhb stars are less luminous and therefore closer in distance to us . establishing the evolutionary stage of hvs7 is critical because its radial velocity is marginally consistent with it being a bhb runaway star bound to our galaxy @xcite . the lack of significant radial velocity variations for hvs7 suggests it is not a binary , nor a pulsator . slowly pulsating main sequence b - type stars typically show radial velocity variations of @xmath32 20  km  s@xmath1 in amplitude @xcite , while bhb stars appear to be stable , as they fall outside the rr lyrae instability strip @xcite . the long term radial velocity stability of hvs7 , combined with the metal abundance anomalies ( see  [ sec : hvs7 ] ) , hint towards hvs7 being a bhb star . its @xmath3 ( 60@xmath33 km  s@xmath1 ) is higher than typically found for bhb stars ( @xmath29 km  s@xmath1 ) , although rotators with @xmath3 up to @xmath34 km  s@xmath1 have been observed ( e.g. * ? ? ? * ; * ? ? ? * ) . the true nature of hvs7 as a bound bhb star will have to be disentangled by astrometry .    for hvs8 we detect a radial velocity variation of 23 km  s@xmath1 , consistent with a pulsating main sequence b - type star . we can not discard the possibility of hvs8 being a binary , although the system would have a very low mass - ratio , since there is no evidence of lines from a companion in the spectrum . the star is most likely a main sequence slow pulsator , like hvs1 @xcite . the low s / n of our spectrum does not allow to test for metal abundance anomalies in hvs8 , however , the high rotational velocity of this star will make its abundance analysis difficult , even with higher s / n spectra . we have derived @xmath3 , @xmath13 and @xmath14 for hvs7 and hvs8 , two of the ten currenty known hvss . their @xmath13 and @xmath14 are consistent with the stars being late b - type , as initially classified by @xcite using photometric color indexes . hvs7 has a projected rotational velocity @xmath3 = @xmath35 km  s@xmath1 , while for hvs8 @xmath3 = @xmath36 km  s@xmath1 . these measurements provide the first direct observational test to the prediction by @xcite , who suggests that hvss ejected via hills mechanism should rotate systematically slower ( @xmath12 km  s@xmath1 ) than single stars of the same spectral type ( @xmath37 km  s@xmath1 ; * ? ? ? if the hvss have fast rotational velocities typical of single b - type stars in the field , other ejection mechanisms , such as three - body encounters of single stars with @xmath32 @xmath38@xmath39 @xmath40 imbhs , with a binary mbh , or with @xmath32 10 @xmath40 stellar - mass black holes orbiting the galactic smbh , have to be invoked . the @xmath3 values of hvs7 and hvs8 are lower limits to their true rotational velocities , imposed by the inclination angle of the rotation axis of the stars . if the inclination of the rotation axis of hvs7 is low , its rotational velocity could in principle be much higher . in that case both targets are inconsistent with hansen s prediction for hills scenario . however , a sample of only two @xmath3 measurements is not enough to conclusively discern between the different scenarios proposed and more @xmath3 measurements are necessary . statistical tests performed by @xcite conclude that a sample of 25 or more hvss will be needed to distinguish between scenarios at a @xmath41 95@xmath42 confidence level . we also detect abnormal enhancement and depletion effects in the strength of some of the metal lines of hvs7 . these anomalies , together with the apparent lack of pulsations are consistent with hvs7 being a bhb star . its @xmath3 is also marginally consistent with it being a fast - rotating bhb star , if the inclination angle of the star s rotation axis is close to 90 degrees . however , confirmation that hvs7 is a bhb star will not be possible until its proper motion is accurately measured . finally , we find evidence of radial velocity variations in hvs8 consistent with a pulsating main - sequence b - type star nature . additional radial velocity measurements and time - series precision photometry will confirm this detection .                                                                                                 minimization result for full spectrum of hvs7 . _ left : _ contour plot of @xmath14 vs. @xmath13 for a fixed @xmath3 of 60 km  s@xmath1 ; contours correspond to @xmath43 ; crosses to the models in the kurucz grid . _ @xmath16 vs. @xmath3 for the best fit values @xmath13 = 12,000 k and @xmath14 = 3.50 dex . @xmath16 = 1 shows the statistical 1@xmath22 error of the fit ; the result is @xmath44 km  s@xmath1 . a similar analysis was done for hvs8.,width=624 ]
we measure the projected rotational velocities of the late b - type hypervelocity stars hvs7 and hvs8 from high resolution spectroscopy to be 60 @xmath0 17 km  s@xmath1 and 260 @xmath0 70 km  s@xmath1 . 
 the slow rotation of hvs7 is in principle consistent with having originated in a binary system , assuming a high inclination angle of the stellar rotation axis . 
 however , the fast rotation of hvs8 is more typical of single b - type stars . 
 hvs8 could have therefore been ejected by a mechanism other than that proposed by hills . 
 we also estimate the effective temperatures and surface gravities for hvs7 and hvs8 and obtain an additional measurement of their radial velocities . 
 we find evidence in support of a blue horizontal branch nature for hvs7 , and a main sequence nature for hvs8 .