A New, Bright, Short-Period, Emission Line Binary in Ophiuchus

0.42 MB·English

by  M. A. Stark

A New, Bright, Short-Period, Emission Line Binary in Ophiuchus1
M. A. Stark2
Department of Physics and Astronomy, University of Wyoming, Dept. 3905, 1000 E. University Ave.,
Laramie, WY 82071
8
[email protected]
0
0
2
Richard A. Wade

n
Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab,
a
J University Park, PA 16802

8 [email protected]
1

] John R. Thorstensen, Christopher S. Peters
h
p Department of Physics and Astronomy, 6127 Wilder Laboratory, Dartmouth College, Hanover, NH
- 03755-3528
o
r
t
s Horace A. Smith, Robert D. Miller
a
[ Department of Physics and Astronomy, Bio-Physical Sciences Bldg, Michigan State University, East

Lansing, MI 48824-1116
1
v
3
E. M. Green
6
9 Steward Observatory, University of Arizona, Tucson, AZ 85721-0065
2
.
1
0
8
0 ABSTRACT
:
v The 11th magnitude star LSIV−08◦3 has been classified previously as an OB star in the
Xi LuminousStarssurvey,oralternativelyasahotsubdwarf. Itisactuallyabinarystar. Wepresent
spectroscopy,spectroscopicorbitalelements,andtimeseriesphotometry,fromobservationsmade
r
a attheKittPeakNationalObservatory2.1m,StewardObservatory2.3m,MDMObservatory1.3m
and2.4m,Hobby-Eberly9.2m,andMichiganStateUniversity0.6mtelescopes. Thestarexhibits
emissionof varyingstrengthinthe coresofH andHeI absorptionlines. Emissionis alsopresent
at 4686˚A (He II) and near 4640/4650˚A (N III/C III). Time-series spectroscopy collected from
2005 July to 2007 June shows coherent, periodic radial velocity variations of the Hα line, which
weinterpretasorbitalmotionwitha periodof0.1952894(10)days. High-resolutionspectrashow
that there are two emission components, one broad and one narrow, moving in antiphase, as
mightarisefrom anaccretiondisk andthe irradiatedface of the mass donorstar. Lesscoherent,
low-amplitudephotometricvariabilityisalsopresentonatimescalesimilartothe orbitalperiod.
Diffuse interstellar bands indicate considerable reddening, which however is consistent with a
distance of ∼100–200 pc. The star is the likely counterpart of a weak ROSAT X-ray source,
whosepropertiesareconsistentwithaccretioninacataclysmicvariable(CV)binarysystem. We
◦
classify LSIV−08 3 as a new member of the UX UMa subclass of CV stars.
Subject headings: binaries: close — novae, cataclysmic variables — stars: emission-line — stars: indi-
vidual(LSIV−08◦3) — stars: variables: other
1
1. Introduction approximately 4600–6750˚A and 6400–8900˚A at
∼1.3˚A pixel−1. The features observed in the
◦
ThestarLSIV−08 3hasV =11.475,(B−V)=
GoldCam spectra include broad absorption lines
+0.190 (Reed & Niemczak 2000), and was first with emission cores in Hα, Hβ, and He I 5875˚A;
classified “OB” in the Luminous Stars in the emission from He I 6678˚A, He II 4685˚A, and
NorthernMilkyWaycatalog(Nassau & Stephenson N III/C III ∼4640/4650˚A. The emission compo-
1963). Lateritwasreclassified“sdB”byKilkenny & Busse
nents were observed to vary in both strength and
(1992)basedonReticonspectrogramsandStro¨mgren
profile in as little as ∼30 minutes (Fig. 1; see also
photometry. Additionalphotometryofthisobject
§2.2).
is available in the literature (Reed et al. 1998;
All GoldCam spectra were processed using
Reed 1998; Reed & Niemczak 2000), but no ad-
standard IRAF3 routines. Unfavorable observ-
ditional spectroscopic observations have been re-
ing conditions prevented reliable flux calibrations
ported. Astrometricandphotometricdataforthis
on mostnights, so we discuss only the normalized
star, collected from the literature, are reported in
spectra. The continua were fitted by hand using
Table 1.
splines. The equivalent widths (EWs) of features
◦
We obtained spectra of LSIV−08 3 as part of
of interest were measured in each spectrum; the
ourprogramoncompositespectrumhotsubdwarf
wavelength interval encompassed by each feature
stars (Stark & Wade 2003, 2005; Stark 2005;
was determined by eye, noting where the line de-
Stark & Wade 2006), owing to its unusually red
viated from the continuum. For lines with broad
V −Ks andJ−Ks colors as observedin the Two absorptionwingsandcoreemission,twoEWmea-
Micron All-Sky Survey (2MASS; Skrutskie et al.
surementsweremade. ThefirstisthetotalEWof
◦
2006). We found that LSIV−08 3 exhibits broad
thewholeline(absorptionandemissionincluded).
absorptionlines with core emission, which is vari-
ThesecondEWreferstojusttheemissioncompo-
able in both strength and profile shape. We
nent, measured above the point where the emis-
obtained time-series photometry and additional
sion emerges from the absorption wings, giving a
spectroscopy in order to correctly classify this
lower limit for the “true” emission EW. Reported
star.
in Table 3 are the average EWs and their for-
mal errors (not including any systematic errors
2. Observations
or accounting for variability). Also reported are
the number of measurements, their standard de-
2.1. Low-Dispersion Spectra
viation, minimum and maximum values, and the
2.1.1. Kitt Peak National Observatory 2.1m difference between the maximum and minimum.
Telescope (Note that EW> 0 indicates absorption, while
EW<0 indicates emission.)
◦
LSIV−08 3 was observed from the Kitt Peak
Significant reddening of this object is ev-
NationalObservatory(KPNO)2.1mtelescopeus-
idenced by strong interstellar Na I D lines
ing the GoldCam spectrograph during 2003 June
and several diffuse interstellar bands (DIBs).
and September. A journal of observations is
Schlegel, Finkbeiner, & Davis (1998) report total
given in Table 2. All exposures were between
galactic reddening of E(B−V)≈+0.48along the
5 and 20 minutes long. Two different spectro-
◦
line of sight to LSIV−08 3. The reddening and
graph configurations were used, which covered
distance are discussed in §3.2.
1Based in part on observations obtained with the
2.1.2. Steward Observatory 2.3m Telescope
Hobby-EberlyTelescope,whichisajointprojectoftheUni-
versityofTexasatAustin,thePennsylvaniaStateUniver- ◦
Aflux-calibratedspectrumofLSIV−08 3,cov-
sity,StanfordUniversity,Ludwig-Maximillians-Universita¨t
Mu¨nchen,andGeorg-August-Universita¨tGo¨ttingen. ering 3620–6895˚A at ∼3˚A pixel−1 dispersion
2VisitingAstronomer,KittPeakNationalObservatory,
National Optical Astronomy Observatory, which is oper- 3IRAF is distributed by the National Optical Astronomy
atedbytheAssociationofUniversitiesforResearchinAs- Observatories, which are operated by the Association of
tronomy, Inc. (AURA) under cooperative agreement with Universities for Research in Astronomy, Inc., under coop-
theNationalScienceFoundation. erativeagreement withtheNationalScienceFoundation.
2
(∼9˚Aresolution),wasobtainedonUT2002April telescope and modular spectrograph, equipped
3 at the Bok 2.3m telescope on Kitt Peak, Ari- with a SITe 20482 CCD; this gave 2˚A pixel−1
zona,usingtheB&Cspectrographwithathinned, dispersion from 4300 to 7500˚A, with severe vi-
back-illuminated 1200 × 800 Loral CCD. This gnetting towardthe ends of this range. The spec-
“pre-discovery”spectrum is shown in Figure 2. It tral resolution of this setup was slightly better
extendstoshorterwavelengthsthentheGoldCam than that of the Mark III. Some of these expo-
observations, including the Balmer lines up to sures were taken in bright twilight and bracketed
the series limit. A second spectrum was obtained with comparison exposures; for those taken away
with the same instrument setup on UT 2007 May from twilight, the night-sky features were used
8 and was flux-calibrated in a homogeneous way to find the zero point shift. Extensive internal
with the first; the overall spectral energy distri- checks show that this procedure worked well with
bution is very similar, with the flux level differing this setup. Aliasing related to daily cycle count
by about 3%. The H emission lines were weaker uncertainties was suppressed by pushing to large
in 2007, with the emission reversal in Hγ virtu- hour angles;the target’sbrightnessmade it possi-
ally disappearing and the height of Hα emission ble to obtain good signal-to-noise spectra even at
abovethe continuumdiminishing by a factorof2. large airmass.
◦
Kilkenny & Busse(1992)classifiedLSIV−08 3as Flux standard stars were observed to calibrate
“sdB” based on uvby photometry and 2˚A resolu- the instrument response, when the twilight sky
tion Reticon spectroscopy covering 3900–4600˚A, was reasonably clear. Experience suggests that
so Hβ was not observed, and weak emission in the zero point of the MDM spectra should be ac-
the absorption core of Hγ might easily have been curate to about 20%. Exposures of bright O and
overlooked or even absent. The spectral energy B stars were also obtained and used to map and
distribution (SED) is quite blue, even despite the correct for the telluric absorption features.
reddening evidenced by the DIBs. The SED is
An average flux-calibrated spectrum was con-
further discussed in §3.
structed from the 1.3m data taken in 2005 June
and July. Using the IRAF sbands task and the
2.1.3. MDM Observatory 1.3m and 2.4m Tele-
passband tabulated by Bessell (1990), we find a
scopes
synthetic V magnitude of 11.47. The MDM 1.3m
Time-series spectra of LSIV−08◦3 were ob- average spectrum is very similar to the Steward
tained at the MDM Observatory4 on Kitt Peak, 2.3mspectrum from 2002April in the wavelength
Arizona. Table 4 gives a journal of observations. region common to both, except the overall flux
Most of the MDM spectra were obtained using level is about 10% higher, and the emission lines
the MarkIII spectrographanda SITe10242 CCD are more evident (the Hβ emission reversal pro-
detector mounted on the McGraw-Hill 1.3m tele- trudes slightly above the continuum; the He I
scope. A600linemm−1grismgave∼2.3˚Apixel−1 λ6678 line is in emission at about 15% of the in-
dispersionoverthe range4660–6900˚A, with a full tensity of Hα).
widthhalfmaximum(FWHM)resolutionof4.1˚A.
2.1.4. Radial Velocity Variations
All stellar exposures were bracketed with spectra
of Hg, Ne, and Xe comparison lamps, in order
All ofthe MDM spectra showHα emission. To
to track the spectrograph flexure. (In 2004 June,
measuretheradialvelocity(RV),weconvolvedthe
an attempt was made to use the λ5577 night-sky
line profile with the derivative of a Gaussian and
emission feature to set the wavelength zero point;
took the line center to be the zero of this convo-
unfortunately, this did not work well, and those
lution (Schneider & Young 1980). The width of
spectra are excluded from the present analysis.)
the convolving function was optimized for a line
A few MDM spectra are from the Hiltner 2.4m with a FWHM of 11.5˚A, which is approximately
the observed line width. We searched the veloc-
4MDM Observatory is operated by a consortium consist- ity time series for periodicities using a “residual-
ingofDartmouthCollege,ColumbiaUniversity,OhioState
gram” technique (Thorstensen et al. 1996), which
University, the University of Michigan, and Ohio Univer-
isespeciallyeffectivewhenthemodulationisaccu-
sity.
ratelysinusoidal. Becausethedataspanhundreds
3
◦
of days, we were careful to use a sufficiently fine shortened exposures obtained as LSIV−08 3 ap-
gridoftrialfrequencies. Theperiodsearchyielded proachedtheendoftheHETtrack. Threeorfour
a single, uniquely-defined frequency near 5.12 cy- separate exposures were obtained in quick suc-
cle d−1; a least-squares fit to the velocities of the cession on each of four nights in 2004 and three
form nights in 2007 (see Table 5 for a journal of obser-
2π(t−T )
0 vations). The typicalsignal-to-noiseratiowas100
v(t)=γ+Ksin
(cid:20) P (cid:21) at 5822˚A (range 75–120). Th-Ar lamps and flat
then yielded a preliminary period of 0.195290 d, field exposures were taken either before or after
with a velocity semiamplitude of 80 km s−1. the sequence of exposures each night.
Twenty-three additional RVs were used to ex- These spectra were processed with standard
tend the span of RV observations and to improve IRAF routines. At the dispersionof the HRS, the
the precision of the ephemeris. These were mea- absorption components of the Balmer lines in the
◦
sured using the same 11.5˚A FWHM Gaussian- LSIV−08 3 spectrum are roughly as broad as a
derivative technique, applied to high-resolution single spectral order. Thus, a pseudo-continuum
spectra from the Hobby-Eberly Telescope (HET) was fitted to the broadH absorptionprofiles, and
obtained in 2004 and 2007 (see §2.2 below). The each spectrum was normalized using this fit. The
final heliocentric Hα RV ephemeris, based on 163 data were then rebinned in uniform heliocentric
points, is given by: wavelength bins. Figure 4 shows the normal-
ized Hα core emission profiles at several key or-
T = HJD 2453628.6389±0.0016, bital phases. As the phase varies, the profile does
0
not simply shift back and forth in velocity, but
P = 0.1952894±0.0000010d,
rather changes shape. To the eye, a decomposi-
K = 77±4 km s−1, and
tion into a broad component and a narrow com-
γ = −44±3 km s−1. ponent moving in anti-phase is suggested. This
is made more evident in Figure 5, which gives a
trailed-spectrogramrepresentationofallthe data.
The rms residual between the data and the fit-
The measured EW of the Hα emission (above
ted curve is 17 km s−1. Figure 3 shows the RVs
the pseudo-continuum) was roughly constant at
folded on the period, with the best-fitting sinu- ∼3.0˚A in 2004, but was lower in 2007, averag-
soidsuperposed. TheperiodicityintheHαRVsis ing 1.7, 2.1, and 1.8˚A on 2007 April 19, June 8,
coherentoverthreeobservingseasons,andthe ve-
and June 22 respectively. To make Figs. 4 and 5,
locities admit no possibility of a cycle count error
we have “stretched” each spectrum to achieve a
— the data are consistent with a binary orbit. uniform emission EW of 3.0˚A, keeping the con-
tinuum level at unity. In the trailed spectrogram
2.2. High-Resolution Spectra
(Fig.5),eachspectrumhasthenbeenmadetofilla
Follow-up observations were taken at the phase range of 0.05 cycles, centered on the phase
HET using the High Resolution Spectrograph bin (bin width = 0.01 cycles) nearest to the or-
(HRS; Tull 1998). Owing to the declination of bital phase at mid-observation. Note that data
LSIV−08◦3, it can only be observed by the HET from 2004 and 2007 have been intermingled, and
for about one hour per night as it is transit- different orbital phases were observed in different
ing, from mid-April through mid-July. Queue years. The change in EW from one year to the
observations of LSIV−08◦3 were made with the other may signal a change in the behavior of the
HRS using 2′′ fibers (one object fiber and two profilevsorbitalphase,sowecautionthatquanti-
sky fibers) and 2×3 on-chip binning, resulting in tativeconclusionsaboutthesedistinctnarrowand
R ≡ λ/∆λ ≈ 30,000, covering the wavelength re- broadvelocitycomponents,basedonthesefigures,
gion from Hβ to Hα. (Unfortunately, the λ5780 are tentative.
DIB falls into the gap between the “red” and The phase of the broad component in the Hα
“blue” CCDs with this setup.) Exposure dura- emission profile is given by the RV ephemeris de-
tions were 10 minutes, with the exception of the rived in §2.1.4, with the narrow component mov-
first observation of 20 min duration and a few ing in anti-phase. Thus, the Gaussian-derivative
4
methodusingFWHM=11.5˚Adescribedin§2.1.4 2.3. Photometry
likely gives a “diluted” RV semiamplitude for the
2.3.1. Michigan State University 0.6m Telescope
broad component. Given the possible change in
the profile behavior between 2004and 2007noted DifferentialV bandphotometry of LSIV−08◦3
above, we did not attempt to decompose each was obtained using the Michigan State Univer-
observed profile or otherwise make formal two- sity 0.6m telescope equipped with an Apogee
component fits to the high-resolution data. A Ap47p CCD camera. Aperture photometry for
rough estimate gives FWHM ∼8–10and ∼3˚A for LSIV−08◦3 was obtained relative to the star
the broad and narrow components. Estimates of TYC 5642−00482−1, which has BT = 12.389,
the RV semiamplitudes are Kbroad ∼ 120 and VT = 11.183. (This star is significantly redder
Knarrow ∼ 90 km s−1, with errors of perhaps than the program star, but there is no blue star
20 km s−1. of comparable brightness to the target within the
We tentatively identify the broad emission as CCD field of view.) Observations were made on
arising from an accretion disk around a compact twelve nights: UT 2004 July 26; 2004 August 1,
object and the narrow emission as arising from 7, 9, 16, 22, 31; 2004 September 18; 2005 July
the mass donating star in a cataclysmic binary. If 15, 20, 23; and 2006 May 27. Exposure times
the narrow emission arises on the hemisphere of were typically 30 s, varying somewhat depending
the mass donor that faces the disk (see §3.1), one on observing conditions. Observing run lengths
mightexpectthe EWofthenarrowcomponentto variedfromasshortasaboutanhourto6.3hron
vary, such that the emission EW is largest when 2006 May 27, this being about the maximum run
themassdonorisatsuperiorconjunction,whichis lengthpossibleatreasonableairmassfromthelat-
orbital phase 0.5 in our convention. We searched itude of the observatory. Calibrations were done
forthiseffectusingthelow-resolutionMDMspec- using standard procedures and sky flats. Since
tra from the most extensive data set (2005 July), observations were only taken in the V filter, no
but did not find any firm indication of an EW transformations to the standard system involving
variation with orbital phase. The limit on the colortermscouldbe attempted. Thetypicalerror
variation in the total EW is 15–20% full ampli- of a single observation is ±0.02 mag.
tude. It is not possible to decompose these low- The photometry shows LSIV−08◦3 to be vari-
resolution spectra into separate components. The ableonbothshort(flickering)andlongtimescales,
high-resolutionHRSspectraareunfortunatelynot with no indication of eclipses. By itself, the pho-
suitable for this test, since all data collected near tometry suggests no clear or consistent period of
orbitalphase0.5comefromthe2007observations, variation — folding the data on numerous trial
and all data colleced near orbital phase 0.0 come periods showedmanypossible aliasperiods. Fold-
from 2004. While there is a tantalizing difference ing the data on the period derived from the RV
in totalEW betweenthese two observingseasons, observations(§2.1.4)doesshowweakmodulations
we cannot know whether it is related to orbital or waves, perhaps multi-periodic, in the bright-
phase;amoreconcentratedobservingcampaignis ness of LSIV−08◦3 (Fig. 6). The rapid modula-
needed. We note that there need not be a large tion in ∆V that is seen in the 2005 Jul 23 data,
modulation of the EW of the narrow component, ofamplitude∼0.03mag andlasting severalcycles
iftheorbitalinclinationofthebinaryismoderate. overthephaseinterval∼0.2–0.6,isnotseeninthe
The Hβ emission line profile shows behavior photometriccheckstars;thismaybesimplyanin-
similar to Hα, with the narrow component being stance of strong flickering. The median light level
perhaps more pronounced (slightly higher inten- is about∼0.06mag brighterin2005–2006than in
sity compared with the broad component). Sim- 2004. Additional time-series photometry, from a
ilar behavior is also reflected in the He I λ5876 more southern site to permit longer nightly runs,
emission profile, although it is weaker. wouldbedesirableincharacterizingthelightcurve
and searching for persistent periodic signals.
2.3.2. Northern Sky Variability Survey
◦
LSIV−08 3 was detected as a variable star
5
in the Northern Sky Variability Survey (NSVS; subclass. These systems show persistent broad
◦
LSIV−08 3 ≡ NSVS 16408817; Wo´zniak et al. Balmer absorption-line spectra. Ironically, these
2004). The NSVS record shows 92 observations lines were once thought to indicate pressure-
ofthis star. The typicalerrorofa singlemeasure- broadening in the atmosphere of a hot subdwarf
mentis∼0.02mag. Thedataaresparse,collected orwhitedwarf—e.g.,Walker & Herbig(1954)—
on45nightsduringtheinterval1999Aprilto2000 and only later were reinterpreted as arising from
March, with no more than four observations per an optically thick accretion disk, where they are
night. AlightcurvefoldedonourRVperiodshows (at least in part) kinematically broadened.
no phase-dependent pattern. The median unfil- UX UMa stars also exhibit a wide range in
tered optical magnitude is 11.855 with a scatter strengths of the emission line components. Emis-
of 0.052 mag. The total range of observedmagni- sion from combinations of H I, He I, He II, N III,
tudes is 11.698±0.026to 11.977±0.068. The data and C III have all been observed at a variety of
exhibitvariationsinmedianbrightnessatthe0.05 strengths in various UX UMa stars, or variable
mag level with characteristic time scales of 20– over time in a single star of the class. Emis-
40 days. Similar to the epoch-to-epoch variations sion is seen in all of these lines in LSIV−08◦3,
seen in the MSU observations, this suggests that and the features are consistent with those ob-
the system has been consistent in its photometric served in the UX UMa stars RW Sex and IX Vel
behavior over the period of 1999-2006. (Beuermann et al. 1992; Beuermann & Thomas
1990, respectively). For UX UMa stars, the
3. Discussion Balmer decrement in the emission components is
steeper than in the absorption lines. This can re-
3.1. Novalike CV Interpretation
sultin asituationwhere Hα is purelyin emission,
Wehavealreadysuggested(§2.1.2)thatLSIV−08◦3 while highermembersshowprogressivelystronger
could have been misclassified as a hot subdwarf (less filled-in) absorption troughs. This effect is
◦
(sdB) star, given only photometry and spec- seen in LSIV−08 3 (Fig. 2).
troscopyconfinedto wavelengthsbluewardofHβ. Narrow Balmer emission has been found in
Indeed, the dwarf nova FO Per (aliases: RL 92 many UX UMa systems, superimposed on the
and RWT 92) was classified by Chromey (1979) broaderemissionfromthedisk. Thisnarrowcom-
asansdBstar,when,unbeknownsttohim,itmust ponent is attributed to irradiation of the atmo-
have been near maximum light in its outburst cy- sphere of the secondary star, with reprocessing of
cle. TwoStewardObservatory2.3mspectraofFO thelighttoBalmeremission. Asdiscussedin§2.2,
Per, also near maximum light, obtained in 2004 the asymmetryandvariationsin the emissionline
Dec and 2006 Dec with the identical setup as de- profile shape (Figs. 4 & 5) suggest the presence
scribed in §2.1.2, show a spectrum that is almost of such a component, moving in anti-phase to the
indistinguishable from a somewhat reddened sdB broad component, as required for this interpreta-
star, except for emission cores in Hα, Hβ, and, tion.
very faintly, in the next few higher order Balmer
The orbital period of ∼ 0.195d (∼ 4.7 hours)
lines, i.e., atalmostexactlythe same levelasseen ◦
determinedforLSIV−08 3iscomparabletoother
◦
in our 2007 May spectrum of LSIV−08 3.
UX UMa systems.
◦
The SED ofLSIV−08 3 is considerablyredder
thanthoseofknownsinglesdBstars. Dereddening 3.2. Distance and Reddening
bytheequivalentofE(B−V)∼0.6ormorewould
◦
LSIV−08 3 lies at galactic coordinates (l,b)=
be required to bring the Steward SED at optical
◦ ◦
(11.0,+20.8). The Schlegel et al. (1998) redden-
wavelengths into agreement with sdBs that have
ing maps show that the total line-of-sight extinc-
T in the range 24,000 — 35,000 K. Likewise,
eff tion varies on small angular scales in this direc-
theE(B−V)neededtobringthe2MASSinfrared
tion. We attempted to infer the E(B−V) color
colorsintotherangeofsinglesdBstarsis0.6mag
excess directly from the strength of the DIBs, us-
or more.
ing the linear relations, W = a E(B−V), estab-
◦ 0
On the other hand, LSIV−08 3 shares many
lished by Herbig (1975). The λ5780 DIB has an
properties with novalike variables ofthe UX UMa
6
EW of 0.32±0.03˚A, suggesting E(B−V) ≈ 0.50 Voges et al. 1999). The count rate in the po-
magwithanuncertaintyofatleast10%,if weare sition sensitive proportional counter (PSPC)
measuring the narrow component of this feature was 0.11±0.02 s−1, yielding an estimated 0.1–
only, or E(B−V)≈0.24 mag if we are measuring 2.4keV flux of 1.5×10−12 erg cm−2 s−1. Taking
◦
the sum of the narrow and broad DIBs (the lat- LSIV−08 3 to be the optical counterpart, the X-
ter is more likely). The λ4430 DIB has an EW of ray/optical flux ratio is given by logfX/fopt ≈
0.26±0.04˚A, corresponding to E(B−V) ≈ 0.12 −2.0.
with atleast15%uncertainty. Some ofthe uncer- This source was also observed serendipitously
tainty arises, depending on whether “all-sky” or by ROSAT (1WGA J1656.4−0834; White et al.
“Sco-Oph” values of a0 are adopted. Both DIBs 2000)duringapointedobservationofanothertar-
imply reddening less than the full Schlegel et al. get. Inthis∼8000 sexposure,150netcountswere
(1998) column: E(B−V)=+0.47. collected, and the count rate was 0.02±0.002 s−1
The heliocentric RV of the interstellar Na I D (5 times lower than in the RASS), correspond-
lines is −14.5 km s−1 (average of D1 and D2). ing to fX ≈ 3.0 × 10−13 erg cm−2 s−1 and
Most of this can be explained by the reflex of logfX/fopt ≈ −2.6. A best-fitting blackbody
the Sun’s space motion with respect to the Local spectrum has kT ≈ 0.2 keV with an intervening
Standardof Rest, so kinematic informationabout hydrogencolumndensityof1.3×1021cm−2,corre-
thedistanceoftheabsorbingmaterialbetweenthe spondingtoE(B−V)≈0.2. Theunabsorbedflux
Sun and LSIV−08◦3 is unavailable. is F0.5−2 keV ≈2.7×10−13 ergs−1cm−2. Fits us-
IfLSIV−08◦3isassumedtobeaUXUMavari- ing a bremsstrahlung or thermal plasma emission
ablestar,howfarawayis it,andis this consistent model would result in a similar kT value, given
with the measured reddening? We adopt an ab- the highly absorbed spectrum and the PSPC’s
solute magnitude MV ≈ +5 and a representative limited energy range and resolution. The inferred
apparent magnitude V = 11.50 for LSIV−08◦3. total X-ray flux is quite uncertain.
We use the standard relation AV = 3.1E(B−V) X-ray emission is not associated with sdB
to compute distance d as a function of assumed stars. On the other hand, CVs of all subclasses
E(B−V). Values between 100 and 200 pc re- show X-ray emission. Verbunt et al. (1997) dis-
sult for E(B−V) in the range 0.0 to 0.5 mag, cuss the CVs detected in the RASS. A typical
smaller distances corresponding to larger redden- luminosity in the 0.5–2.5 keV band for a no-
ings. Ak et al. (2007) have put forward a calibra- valike variable is LX ∼ 1031 erg s−1. The re-
◦
tionofMJ forCVs,usingasinputstheorbitalpe- quired distance for LSIV−08 3 to have this LX,
riod and the extinction-corrected 2MASS J mag- given its modeled flux, is d ∼ 500 pc. Us-
nitude and J −H, H −Ks colors. Again varying ing a 2 keV bremsstrahlung spectrum to model
theassumedE(B−V),wefinddintherange140– the ROSAT counts, Verbunt et al. (1997) find
180 pc by this method (minimum error at fixed logFX/FUV+opt < −2.5 to be typical for nova-
reddening is 40%). Comparing the 2MASS colors likes. Given the uncertainties in the input spec-
◦
of LSIV−08 3 and IX Vel directly, we find a rea- trum and absorption column, we conclude that
◦
sonable match if E(B−V) ≈ 0.10 mag. We con- LSIV−08 3 has X-ray properties consistent with
cludethat,ifE(B−V)intherange0.10–0.20mag membership in the UX UMa subclass of CVs.
canbeaccommodatedwithindistancesof100–200
pc, the colors and magnitudes of LSIV−08◦3 are 4. Summary
consistentwiththoseofaUXUMavariable. Such
Giventhe manysimilaritiesbetweenour obser-
reddening values within such distances are plausi-
◦ vations and other UX UMa novalike variables, we
ble along the line of sight to LSIV−08 3, accord-
◦
proposethatLSIV−08 3shouldbeclassifiedwith
ingtothemapsof,e.g.,Perry & Johnston(1982).
theUXUMastars. Evidencesupportingthisclas-
3.3. ROSAT X-ray Observations sification includes the presence of emission rever-
sals in the Balmer absorption series; other emis-
A weak X-ray source detected in the ROSAT
sion lines including He I, He II, and N III/C III;
All-Sky Survey (RASS) appears to be coinci-
the variability of the emission line strengths; the
◦
dent with LSIV−08 3 (1RXS J165630.2−083442;
7
intrinsically blue continuum; and the mild photo- by, e.g., the SloanDigital Sky Survey,the sky has
metric variability. The key evidence in favor of not been thoroughly explored at V ∼11.
a CV interpretation lies in the periodically mod-
ulated Doppler shifts and profiles of the emission M.A.StarkthanksG.B.Berriman,forhishelp
lines,withaperiodof4.7hr,consistentwithalow- interpretingthe2MASScatalog,andK.T.Lewis,
mass quasi-mainsequence star that fills its Roche A. Narayanan, and K. Herrmann, for assisting
lobe and transfers mass to a luminous accretion with the observations at KPNO. We are grate-
disk around a white dwarf. The inner hemisphere ful to K. T. Lewis for carrying out the fit to the
of the donor star may be irradiated by the ac- ROSAT data, and to L. Townsley, who provided
cretion disk. Reddening and distance estimates helpful interpretation. We also thank the HET
are consistent with the CV interpretation, from resident astronomers M. Shetrone, S. C. Ode-
the standpoint of the expected luminosity of the wahn, and H. Edelmann and telescope operators
system and the distribution of interstellar dust in F.Deglman,M.Soukup,M.Villarreal,andV.Ri-
◦
thedirectionofLSIV−08 3. Finally,weakX-rays ley. TheHobby-EberlyTelescopeisajointproject
are apparently associated with the system, and of the University of Texas at Austin, the Penn-
the X-ray luminosity and X-ray/optical flux ratio sylvania State University, Stanford University,
are consistent with observations of other novalike Ludwig-Maximillians-Universit¨at Mu¨nchen, and
CVs. Georg-August-Universit¨at G¨ottingen. The HET
We refrain from deriving the dynamical mass is named in honor of its principal benefactors,
ratioofthebinaryorindividualmassesofthestars William P. Hobby and RobertE. Eberly. This re-
◦
in LSIV−08 3 from any of the various RV semi- search has been supported by grants from NASA
amplitudes that we have presented above, mainly (including NASA GSRP grant NGT5-50399), the
because the HRS spectra on whichsuchestimates ZaccheusDanielFoundationforAstronomicalSci-
would rely were obtained on widely separated ence, and the Pennsylvania Space Grant Consor-
dates, and the EW of the emission lines varied tium. JRT andCSP gratefully acknowledgefund-
significantly among the various epochs. It is thus ingfromtheNationalScienceFoundationthrough
possiblethatoneorbothsitesoftheemissionvar- grantAST-0307413,andthankthe MDMstafffor
ied as well. Work aimed at determining the mass expertandconscientiousobservingsupport. Holly
ratio is best undertakenwith intensive time-series SheetstooksomeoftheMDMspectra. Asalways,
spectroscopy that can provide full orbital phase we are grateful to the Tohono O’odham for leas-
coverage in one or two nights, to minimize such ing us their mountain so that we may study the
variations. Likewise, we cannot offer a definitive great universe around us. HAS thanks the Cen-
interpretationofthephotometricvariability,given ter for Cosmic Evolution and the National Sci-
◦
our present limited data. LSIV−08 3 thus re- ence Foundation (grant AST-0440061) for their
mains as an attractive and interesting target for support. EMG acknowledges support from NSF
further study. grant AST-0098699. This publication makes use
Despite the apparentline-of-sight extinction to of data products from the Two Micron All Sky
the source, LSIV−08◦3 is one of the brighter UX Survey, which is a joint project of the Univer-
UMastarssofardiscovered. Downes et al.(2001) sity of Massachusetts and the Infrared Processing
list only three “UX Uma” systems (of two dozen and Analysis Center/California Institute of Tech-
catalogued) that are brighter: IX Vel, V3885 Sgr, nology, funded by the National Aeronautics and
and RW Sex. Only seven CVs of any “nova- Space Administration and the National Science
like”sub-type(89systemstotal)arebrighterthan Foundation.
LSIV−08◦3 at their maximum light. Bright, rel- Facilities: KPNO:2.1m(GoldCam),Bok(B&C
atively nearby systems are not only appropriate spectrograph), Hiltner (modular spectrograph),
for follow-up studies, but contribute dispropor- McGraw-Hill (Mark III spectrograph), HET
tionately to studies of the space density andkine- (HRS)
matics of the various subclasses of CVs. Even
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9
Fig. 2.— Flux-calibrated spectrum of
◦
LSIV−08 3, obtained UT 2002 April 3 at
the Steward Observatory 2.3m telescope, in-
Fig. 1.— KPNO 2.1m Goldcam spectra of
◦ cluding higher-order Balmer lines. Absorption
LSIV−08 3 showing the variable nature of the
emission components at Hβ (top), He I 5875˚A bandheads at λ6277 and λ6867 are telluric; ab-
partlyblendedwithinterstellarNaIDlines(mid- sorptionfeatures at λ4430(near the strongerHeI
dle), and Hα/He I 6678˚A (bottom). Dates and λ4471 feature) and λ5780 are diffuse interstellar
bands.
times are UT 2003; times correspond to mid-
exposureandareheliocentric. Phasescorrespond-
ing to the ephemeris in §2.1.4 are also indicated.
Datesthatareitalicizedinparenthesesinthebot-
tom panel do not have corresponding data for Hβ
andHeI5875˚A.Dottedlinesshowthelaboratory
wavelengthsofthelabeledtransitions;thespectra
themselves are plotted using observed (topocen-
tric) wavelengths. Several gaps in the spectra re-
sult from cosmic ray removal.
10