New Close Binary Systems from the SDSS-I (Data Release Five) and the
  Search for Magnetic White Dwarfs in Cataclysmic Variable Progenitor Systems
Draft version October 22, 2018
Preprint typeset using LATEX style emulateapj v. 08/22/09
NEW CLOSE BINARY SYSTEMS FROM THE SDSS–I (DATA RELEASE FIVE) AND THE SEARCH FOR
MAGNETIC WHITE DWARFS IN CATACLYSMIC VARIABLE PROGENITOR SYSTEMS
Nicole M. Silvestri
, Mara P. Lemagie
, Suzanne L. Hawley
, Andrew A. West
, Gary D. Schmidt
, James
Liebert
, Paula Szkody
, Lee Mannikko
, Michael A. Wolfe
, J. C. Barentine
, Howard J. Brewington
Michael Harvanek
, Jurik Krzesinski
, Dan Long
, Donald P. Schneider
, and Stephanie A. Snedden
(Received 2007 February 9)
Draft version October 22, 2018
ABSTRACT
We present the latest catalog of more than 1200 spectroscopically–selected close binary systems
observed with the Sloan Digital Sky Survey through Data Release Five. We use the catalog to search
for magnetic white dwarfs in cataclysmic variable progenitor systems. Given that approximately 25%
of cataclysmic variables contain a magnetic white dwarf, and that our large sample of close binary
systems should contain many progenitors of cataclysmic variables, it is quite surprising that we find
only two potential magnetic white dwarfs in this sample. The candidate magnetic white dwarfs, if
confirmed, would possess relatively low magnetic field strengths (BWD < 10 MG) that are similar to
those of intermediate–Polars but are much less than the average field strength of the current Polar
population. Additional observations of these systems are required to definitively cast the white dwarfs
as magnetic. Even if these two systems prove to be the first evidence of detached magnetic white
dwarf + M dwarf binaries, there is still a large disparity between the properties of the presently known
cataclysmic variable population and the presumed close binary progenitors.
Subject headings: binaries: close — cataclysmic variables — stars: low-mass — stars: magnetic fields
— stars: white dwarfs
1. INTRODUCTION
The evolution of stars in close binary systems leads
to interesting stellar end-products such as cataclysmic
variables (CVs), Type 1a supernovae, and helium–core
white dwarfs (WDs). The period in which an evolved
star ascends the asymptotic giant branch and engulfs
a close companion in its evolving atmosphere, referred
to as the common envelope phase, probably plays a
dominant role in the evolution of these systems and
as yet is poorly understood. The angular momentum
of the system is believed to aid in the eventual ejec-
tion of the common envelope to reveal the remnant WD
and close companion. After the common envelope has
been ejected, gravitational and magnetic braking work
to decrease the orbital separation of the detached system
(de Kool & Ritter 1993). This orbital evolution contin-
ues through to the CV phase. The effect of the common
envelope on the secondary star in these systems is an-
other aspect of close binary evolution which is not well
characterized. Plausible scenarios for the secondary com-
1 Department of Astronomy, University of Washington, Box
351580, Seattle, WA 98195, USA; nms@astro.washington.edu,
mlemagie@u.washington.edu, slh@astro.washington.edu,
szkody@astro.washington.edu, leeman@u.washington.edu,
maw2323@u.washington.edu.
2 Astronomy Department, 601 Campbell Hall, University of
California, Berkeley, CA 94720, USA; awest@astro.berkeley.edu
3 Department of Astronomy and Steward Observatory, Univer-
sity of Arizona, Tucson, AZ 85721, USA; schmidt@as.arizona.edu,
jliebert@as.arizona.edu.
4 Apache Point Observatory, P.O. Box 59, Sunspot, NM
88349, USA; jcb@apo.nmsu.edu, hbrewington@apo.nmsu.edu,
harvanek@apo.nmsu.edu, long@apo.nmsu.edu, sned-
den@apo.nmsu.edu.
5 Mt. Suhora Observatory, Cracow Pedagogical University, ul.
Podchorazych 2, 30-084 Cracow, Poland; jurek@apo.nmsu.edu.
6 Department of Astronomy, Penn State University, PA 16802
USA; dps@astro.psu.edu
panion range from accreting as much as 90% of its mass
during this phase to escaping relatively unscathed from
the common envelope, emerging in the same state as it
entered (see Livio 1996, and references therein).
Recently, studies of close binary systems with WD
companions (see for example Farihi et al. 2005b,a;
Pourbaix et al. 2005; Silvestri et al. 2006) have revealed
yet another puzzling property of these systems. None of
the WDs in close binary systems with low–mass, main se-
quence companions appear to be magnetic (Liebert et al.
2005). Close, non–interacting binary systems with WD
primaries are quite common and are believed to be the
direct progenitors to CVs (Langer et al. 2000, and ref-
erences therein). Magnetic WDs, stellar remnants with
magnetic fields in excess of ∼ 1 MG, comprise only a
small percentage of the isolated WD population (∼ 2%,
Liebert et al. 2005). Note that the 2% magnetic WD
fraction applies to magnitude–limited samples like the
Palomar–Green (Liebert et al. 1988). However, the same
paper notes that magnetic WDs may generally have
smaller radii than non–magnetic white dwarfs, due to
higher mass. In a given volume, the density of mag-
netic WDs may be ∼ 10% of all WDs (Liebert et al.
2003). The SDSS is also a magnitude limited sample
so we assume a similar expected value for the close bi-
naries. Our sample (as discussed in detail in §2) con-
tains 1253 potential close binary systems. Therefore we
assume approximately 24 of these binaries to harbor a
magnetic WD. Possible implications of the small radii
for magnetic WD + main sequence pairs will be dis-
cussed in §5. However, more than 25% of the WDs in
the currently identified CV population are classified as
magnetic, and many have magnetic fields in excess of 10
MG (see Wickramasinghe & Ferrario 2000).
Holberg et al. (2002) have compiled a list of 109 known
http://arxiv.org/abs/0704.0789v1
2 Silvestri et al.
WDs within 20pc (and complete to within 13pc) that
have nearly complete information about the presence
of a companion. Of the 109 WDs in their sample,
19 ± 4 have nondegenerate companions. Table 7 in
Kawka et al. (2007) lists all known magnetic WDs as of
June 2006. Of the magnetic WDs listed in their table,
149 have field strengths identifiable in SDSS-resolution
spectra (BWD ≥ 3 MG). If the magnetic WDs in the
Kawka et al. (2007) sample are assumed to be drawn
from a similar sample then 28 ± 5.3 would be expected
to have nondegenerate companions, and yet none have
been detected in the Kawka et al. (2007) sample. This is
nearly a 5σ deficit in magnetic WDs with nondegenerate
companions.
Holberg & Magargal (2005) looked at the 2MASS
JHKs photometry of 347 WDs in the Palomar–Green
sample. Of the 347 WDs, 254 had reliable infrared mea-
surements of at least J magnitude. Of these, 59 had
excesses indicative of a nondegenerate companion and
another 15 showed “probable” excesses (Liebert et al.
2005). This gives a WD+dM fraction of 23% (definite
excess) and 29% (including all probable excesses). If the
Kawka et al. (2007) sample had the same frequency of of
nondegenerate companions as the Palomar–Green sam-
ple, they should have 34 and 43, respectively. This is
nearly as 6σ deficit!
This apparent lack of magnetic WDs with main se-
quence companions is not restricted to studies of close
binaries. Low resolution spectroscopic surveys of more
than 500 common proper motion binary systems dis-
covered by Luyten et al. (1964); Luyten (1968, 1972)
and Giclas et al. (1971, 1978) revealed no magnetic WDs
paired with main sequence companions in these wide
pairs (Smith 1997; Silvestri et al. 2005). In addition,
Schmidt et al. (2003) and Vanlandingham et al. (2005)
have identified over 100 magnetic WDs in the Sloan
Digital Sky Survey (SDSS, Gunn et al. 1998; York et al.
2000; Stoughton et al. 2002; Pier et al. 2003; Gunn et al.
2006). As discussed by Liebert et al. (2005), this implies
essentially no overlap between the close binary and mag-
netic WD samples.
A new class of short–period, low accretion–rate polars
(LARPS) identified by Schmidt et al. (2005b) may ex-
plain, in part, these “missing” magnetic WD systems.
In these systems, the donor star has not filled its Roche
Lobe. The WD accretes material by capturing the stel-
lar wind of the secondary. These CVs have accretion
rates that are less than 1% of accretion rates normally
associated with CVs. The discovery of these systems
sheds some light on the whereabouts of magnetic WD
binaries, though as Schmidt et al. (2005b) point out,
this still does not explain the apparent lack of long–
period, detached magnetic WD systems. Thought to be
the first detached binary with a magnetic WD, SDSS
J121209.31+013627.7, a magnetic WD with a proba-
ble brown dwarf (L dwarf) companion (Schmidt et al.
2005a) has been shown to be one of these LARP systems
(Debes et al. 2006; Koen & Maxted 2006; Burleigh). To
date, magnetic WDs have only been found as isolated ob-
jects, in binaries with another degenerate object (WD or
neutron star companion), or in CVs; none have a clearly
main sequence companion.
In this study, we investigate a new large sample of close
binary systems in an effort to uncover these “missing”
magnetic WD binary systems. The sample comprises
more than 1200 close binary systems containing a WD
and main sequence star drawn from the SDSS, many of
which were originally presented in Silvestri et al. (2006,
hereafter, S06). We find that only two of the WDs in
these pairs appear to be magnetic. Even if confirmed,
neither of these WDs has magnetic field strength compa-
rable to those observed in the majority of magnetic (Po-
lar) CV systems. We confirm that the current CV and
close binary populations are indeed disparate and show
that more work is necessary to unravel this mystery.
In §2 we introduce the catalog of close binary sys-
tems through the public SDSS Data Release Five (DR5;
Adelman-McCarthy et al. 2007). We discuss our anal-
ysis techniques in §3 and we present our results in §2.
Our discussion and concluding remarks are given in §5
and §6, respectively.
2. THE SDSS CLOSE BINARY CATALOG THROUGH DR5
The combined properties of the majority of close
binaries in this paper are discussed in detail in
Raymond et al. (2003) and S06. The S06 cata-
log was based on a preliminary list of spectroscopic
plates released internally to the collaboration and as
such does not include objects from ∼ 200 plates re-
leased with the final public Data Release Four (DR4;
Adelman-McCarthy et al. 2006). The additional systems
from both DR4 and DR5 do not change the overall re-
sults from analysis performed in S06, hence no new anal-
ysis is presented here. We include this list in its en-
tirety to complete the DR4 catalog introduced by S06
and add over 300 new systems from the now public DR5
(Adelman-McCarthy et al. 2007). This completes the
catalog of close binary systems with a WD identified
through SDSS–I. More close binaries are being targeted
in the SDSS–II (SEGUE) survey which will continue to
increase the sample through 2008.
The list of 1253 potential close binary systems given
in Table 1 includes objects from all plates released with
the public DR5, thereby superseding the S06 DR4 cat-
alog. The technique used to search for these objects is
the same as described in S06. As with that study, we
do not include systems with low signal-to-noise ratios
(S/N < 5) and do not search for systems with non–DA
WDs. We emphasize that our sample is not complete
(or bias free) due to the selection effects imposed by our
detection methods and due to the sporadic targeting of
these objects in the SDSS spectroscopic survey as dis-
cussed in S06. Thus, our sample represents primarily
bright, DAWD +M dwarf binary systems. As evidenced
by Smolčić et al. (2004), there are potentially thousands
more WD + M dwarf binaries observed photometrically
in the SDSS but not targeted for spectroscopy. Our cat-
alog represents an interesting and statistically significant
sampling of these systems, the properties of which can
be used to test models of close binary evolution (see
Politano & Weiler 2006, for example).
The list of plate numbers from which this
sample has been drawn can be found at
http://das.sdss.org/DR5/data/spectro/1d 23/. This
plate list includes both “extra” and “special” plates.
The extra plates are repeat observations of survey plates
taken during normal operation. The special plates are
observations for special programs (e.g. SEGUE, F stars,
http://das.sdss.org/DR5/data/spectro/1d_23/
Magnetic WDs in CV Progenitors 3
Fig. 1.— Example of an M dwarf with excess blue flux (:+dM)
from Table 1. The companion is seen as little more than excess blue
flux in the M dwarf spectrum. Follow-up spectroscopy to resolve
the companion is necessary to rule out the presence of a magnetic
WD. Note: spectrum has been boxcar smoothed with a filter size
of seven.
main sequence turnoff stars, quasar selection efficiency,
etc.) that are not part of the original SDSS–I survey.
The first four columns of Table 1 list the SDSS identi-
fier, the plate number, fiber identification, and modified
Julian date (MJD) of the observation, followed by the
spectral type of the components (determined visually)
where Sp1 represents the blue object and Sp2 is the red
object. Columns 6 and 7 give the J2000 coordinates (in
decimal degrees) for the object. The next 15 columns
give the ugriz PSF photometry (Fukugita et al. 1996;
Hogg et al. 2001; Ivezić et al. 2004; Smith et al. 2002;
Tucker et al. 2006), photometric uncertainties (σugriz),
and reddening (Augriz). The magnitudes are not cor-
rected for Galactic extinction. Column 23 lists the SDSS
data release in which the object was discovered as well as
additional references in the literature. Additional notes
for the objects are listed in column 24.
The objects identified in Table 1 as :+dM are likely
M dwarfs with faint, cool WD companions. The dis-
covery spectra for these objects reveal little more than
excess blue flux at wavelengths shorter than 5000 Å, as
shown in Figure 1. It is possible that some of these pairs
may contain a magnetic WD; however, much higher S/N
spectra are required to adequately characterize the blue
component of these systems.
Similarly, the thirty nine objects identified as WD+:
or WD+:e (see Figure 8 of S06) have either some ex-
cess flux in the red or have emission at Balmer wave-
lengths indicative of a faint, active, low–mass or sub–
stellar companion. The companion to the magnetic WD
in Schmidt et al. (2005a) was first identified by emission
at Hα in the SDSS discovery spectrum. Other than the
emission at Hα this object had no other optical signa-
ture of a companion. We are performing followup obser-
vations using the ARC 3.5–m telescope at Apache Point
Observatory to obtain radial velocities and near–infrared
imaging of these objects to measure the orbital periods
and categorize the probable low–mass companion’s spec-
tral type. We have already confirmed that none of these
systems contain a magnetic WD.
3. THE SEARCH FOR MAGNETIC WDS
Schmidt et al. (2003) and Vanlandingham et al. (2005)
demonstrated that magnetic WDs with field strengths as
low as ∼ 3 MG can be effectively measured using SDSS
spectra. Visual inspection of the systems in our sample
reveals no obvious magnetic WDs in spectra with good
S/N (> 10) (Lemagie et al. 2004). Most are classical
Fig. 2.— A Typical WD+dM System: SDSS
J140723.03+003841.7, the superposition of a DA (hydrogen
atmosphere) WD and a M4 red dwarf star. Hα emission is
visible in many of these systems and is a result of chromspheric
activity on the surface of the M star, perhaps enhanced due to
the influence of the WD. The lack of broad Zeeman absorption
features in the hydrogen lines indicates that the magnetic field
strength of the WD is very low (compare with Figure 4).
WD + M dwarf close binaries as shown in Figure 2. Of
interest are the lower quality spectra, where the features
of the WD are less obvious because of low S/N and/or
contamination by the spectral features of the close M
dwarf companion. These effects make it difficult to iden-
tify small magnetic field effects on the WD absorption
features. Thus, relatively low magnetic fields (BWD < 10
MG) are not easily recognized in the combined spectrum.
3.1. The Simulated Magnetic Binary Systems
Given the difficulties associated with visually identi-
fying features in these systems, we developed a method
to search for the characteristic Zeeman splitting of the
DA WD absorption features that is also sensitive to low
magnetic field WDs. We use a program that attempts
to match absorption features in magnetic DA WD mod-
els (see Kemic 1974b,a; Schmidt et al. 2003, and refer-
ences therein for details on the models) through an it-
erative method of smoothing and searching the stellar
spectrum. To develop a robust program to search for
magnetic WDs in close binaries we first tested our pro-
gram on WDs of known magnetic field strength. We
used the magnetic DA WDs with field strengths between
1.5 MG ≤ BWD ≤ 30 MG from Schmidt et al. (2003)
and Vanlandingham et al. (2005) as our test sample. We
then constructed model spectra at every half–MG be-
tween 1.5 MG ≤ BWD ≤ 30 MG, each with magnetic
field inclinations of 30◦, 60◦, and 90◦. The program was
able to match (using a χ2 minimization) the magnetic
field strength of each of the magnetic WDs to within ±5
MG of the value quoted in Schmidt et al. (2003).
We then constructed a sample of simulated SDSS spec-
tra of magnetic binary systems. The simulated binaries
were created by adding the spectra of magnetic WDs
used in our initial test from Schmidt et al. (2003) and
Vanlandingham et al. (2005) to the M star templates of
Hawley et al. (2002). We first normalized all spectra at
a wavelength of 6500 Å, and then combined them with
flux ratios of 1:4 (WD:M dwarf) to 4:1 to replicate the
range of flux ratios observed in the close binary sample
(see Figure 3)7. This created a sample of binaries which
represent the average brightness and spectral type dis-
tribution of the majority of the systems in Table 1 (i.e.
7 Note that 6500 Å is the midpoint of the SDSS combined blue
and red spectra, as plotted in Figure 3. In reality the SDSS spectra
extend to below 3900 Å and to nearly 10000 Å.
4 Silvestri et al.
Fig. 3.— Comparison of simulated and observed pre–cataclysmic
variable (PCV) systems. Left Hand Column: Simulated magnetic
PCVs produced by adding WD spectra from Schmidt et al. (2003)
to M dwarf spectra from Hawley et al. (2002) with brightness ratios
as specified at 6500 Å. Right Hand Column: Observed PCVs from
Silvestri et al. (2006).
Fig. 4.— A Simulated System. Top Left Panel: A 13 MG mag-
netic WD from Schmidt et al. (2003). Top Right Panel: Template
M4 dwarf star from Hawley et al. (2002). Bottom Panel: addition
of the magnetic WD and template M dwarf, assuming equal flux
density at 6500 Å.
DA WDs and M0–M5 dwarfs).
Figure 4 is an example of one of the simulated magnetic
binary systems. The upper left hand panel is the SDSS
spectrum of a 13 MG magnetic WD, the upper right
hand panel is the spectrum of a template M4 dwarf star.
The bottom panel is the addition (superposition) of the
two spectra with a flux ratio of 1:1 at 6500 Å. As shown,
this WD with a relatively moderate magnetic field, when
combined with the spectrum of an average M dwarf, is
clearly detected at the resolution of the SDSS spectra (R
∼ 1800).
3.2. Results from the Simulated Systems
We found that detecting the presence of a WD mag-
netic field depends most strongly on the spectral type
and relative flux of the M dwarf companion. Due to the
selection effects of the close binary sample (see S06 for
details), the majority of the M dwarfs in these binaries
have spectral sub–types between M0–M4. In SDSS spec-
tra, early M dwarf spectral types contribute nearly as
much flux in the blue portion of the spectrum (4000–7000
Å) as they do in the red (7000–10000 Å). The spectrum
of the blue magnetic WD is then superimposed onto the
numerous blue molecular features of the M dwarf. This
makes the small absorption features stemming from the
subtle influence of a weak magnetic field difficult to de-
tect.
We plot a subset of our simulated pairs to demon-
strate some of these issues in Figure 5 and Figure 6.
In Figure 5 we selected four early–type template M
dwarfs (WD+M0 = open squares, WD+M1 = open
circles, WD+M2 = open triangles, and WD+M3 =
Fig. 5.— Left Hand Panel: Subset of the simulated binary sys-
tems comprised of early–M dwarfs from Hawley et al. (2002) paired
with magnetic WDs and literature values from Schmidt et al.
(2003) and Vanlandingham et al. (2005) with BWD ≤ 10 MG.
Right Hand Panel: Same M dwarfs from Left Panel paired with
magnetic WDs with BWD ≥ 10 MG. The measured values are
from our program. In both panels, the filled triangles represent
single WDs, open squares are WD+M0, open circles are WD+M1,
open triangles are WD+M2, and crosses are WD+M3. The solid
line has a slope of one and the dashed lines are ±5 MG. Refer to
§ 3.2 of the text for details.
Fig. 6.— Left Hand Panel: Subset of the simulated binary sys-
tems comprised of late-M dwarfs from Hawley et al. (2002) paired
with magnetic WDs and literature values from Schmidt et al.
(2003) and Vanlandingham et al. (2005) with BWD ≤ 10 MG.
Right Hand Panel: Same M dwarfs from Left Panel paired with
magnetic WDs with BWD ≥ 10 MG. The measured values are
from our program. In both panels, the filled triangles represent
single WDs, open squares are WD+M4, open circles are WD+M5,
open triangles are WD+M6, and crosses are WD+M7. The solid
line has a slope of one and the dashed lines are ±5 MG. Refer to
§ 3.2 of the text for details.
crosses) from Hawley et al. (2002) and added them to a
range of magnetic WDs from Schmidt et al. (2003) and
Vanlandingham et al. (2005). The quoted value from
Schmidt et al. (2003) for the magnetic field strength of
each of these WDs represents the “Literature BWD”
value on the x–axis. The “Measured BWD” is the value
returned by the program. Values returned by the pro-
gram that matched the literature values fall along the
solid line. The dashed lines represent ±5 MG of the
literature value. Figure 6 is the same except we add
the same magnetic WDs to later–type M dwarf tem-
plates (WD+M4 = open squares, WD+M5 = open cir-
cles, WD+M6 = open triangles, andWD+M7 = crosses).
The solid triangles represent the tests using the isolated
WD spectra.
In both Figures 5 and 6 the program returns the value
of the single WD to within ∼ ±2 MG for the large ma-
jority of the systems. The uncertainty of the fitted value
and the spread in values increases for magnetic fields of 3
MG or less when the magnetic WD is paired with an M
dwarf of comparable brightness. The flux minima associ-
ated with the Zeeman features for such low field strengths
are just barely resolvable in high S/N spectra of isolated
SDSS WDs (see Schmidt et al. 2003). The added com-
Magnetic WDs in CV Progenitors 5
Fig. 7.— Here, we plot the flux ratio (WD flux/ M dwarf
[dM]) versus the difference between the literature value (from
Schmidt et al. 2003; Vanlandingham et al. 2005) of the magnetic
field strength (BLit) and the measured magnetic field strength
(BMea) as determined from the WD Hα (top panel), Hβ (center
panel), and Hγ (bottom panel) absorption features. Error bars are
from the χ2 fit. Refer to § 3.2 of the text for details.
plexity of the M dwarf molecular features and the gen-
erally lower S/N spectra make it difficult to measure the
magnetic features for low magnetic field strengths. How-
ever, WDs with magnetic fields ≥ 4 MG were easily mea-
sured at all M dwarf spectral types.
In both Figures, the largest discrepancies between the
literature and measured values occur when the WD’s
magnetic field is between 12 MG ≤ BWD ≤ 18 MG;
this is true when the WD is paired with both early– and
late–type M dwarfs. Inspection of the model results in-
dicates that at these field strengths, the Zeeman features
overlap on wavelengths with strong M dwarf molecular
features, causing confusion in the identification of the fea-
ture. However, WD spectra with these and larger field
strengths are quite easily recognized visually so we are
confident that no systems with ≥ 10 MG have escaped
notice, though the exact value of the field strength would
be more uncertain.
In Figure 7, we demonstrate the effect of the relative
flux ratio (WD: M dwarf [dM]) on the identification of the
magnetic field strength of WDs in the simulated binary
sample. The Figure gives the relative flux ratio versus
the difference between the magnetic fields quoted in the
literature and those returned by the program. We use
the same BWD distribution in Figure 7 as used in Fig-
ure 5 and Figure 6. The literature values (BLit) are from
Schmidt et al. (2003) and Vanlandingham et al. (2005).
The three panels show ratios determined using Hα (top),
Hβ (center), and Hγ (bottom). The program consis-
tently returns the quoted BWD as determined from Hβ
until the flux contribution from the M dwarf is nearly
double the flux contribution from the WD. The program
returns the magnetic field from the Hα feature to within
±5 MG until the flux contribution from the M dwarf is
nearly 1.5× the flux from the WD. The BWD as mea-
sured by Hγ is consistently 15–25 MG larger than the
BWD value in the literature at any flux ratio. The con-
tribution of a relatively clean spectral region near Hβ, to-
gether with the fairly strong Zeeman signal at this wave-
length makes Hβ a reliable indicator of WD magnetic
field strength for binaries with flux ratios up to 1:2.
4. TWO POSSIBLE MAGNETIC WDS IN THE DR5 CLOSE
BINARY SAMPLE
The method employed by S06 to split the binary sys-
tem into its two component spectra through an itera-
tive method of fitting and subtracting WD model at-
mospheres and template M dwarf spectra was not used
Fig. 8.— Two potential magnetic DA WD + M dwarf pairs as
identified by our program. The tentative magnetic field strengths
are 8 MG ±5 MG (top) and 3 MG +5/−3 MG (bottom) as deter-
mined from the Hα and Hβ WD absorption features.
on these objects. There are no obviously strong mag-
netic WDs in the sample, suggesting that any possibly
magnetic WDs must possess relatively weak fields. The
subsequent fitting and subtraction of model WDs and
template M dwarfs adds noise to the spectrum which
would make detection of an already weak magnetic field
even more difficult. Also, we would be subtracting a
non–magnetic WD model from the spectrum of a poten-
tially magnetic WD in our attempt to improve the M
dwarf template fit. This adds absorption features where
none actually exist, further corrupting the WD spectrum.
Given these complications, we chose to work with the
original composite SDSS discovery spectra.
Table 2 lists the properties of the only two close bi-
nary systems flagged by our program as containing po-
tential magnetic WDs: SDSS J082828.18+471737.9 and
SDSS J125250.03−020608.1. The first four columns are
the same as for Table 1, followed by the R.A. and
Decl. (J2000 coordinates). The tentative magnetic field
strengths (in MG), inclination of the WD magnetic field
to the line of sight (in degrees) and the spectral types
of the components are listed in Columns 7–9. For each
of these systems the magnetic field strength estimate is
based upon a match to at least two of the three Balmer
features (Hα, Hβ, and/or Hγ) to within ±5 MG of the
model minima. The last six columns give the ugriz pho-
tometry and the SDSS data release for the objects. Refer
to Table 1 for a full listing of photometric errors, redden-
ing and alternate literature sources.
Figure 8 displays the spectra of these two objects,
which have relatively low S/N (∼ 5 at Hα) The iden-
tification of the magnetic field strength was determined
from the Hα and Hβ features in each spectrum, which
upon closer inspection may show some Zeeman splitting.
The best fit model for SDSS J082828.18+471737.9 has a
magnetic field strength of 8 MG and an inclination of 90◦,
while the best fit model for SDSS J125250.03−020608.1
has a magnetic field strength of 3 MG and an inclina-
tion of 90◦. Hβ appears to be distorted in both systems,
indicating a potential broadening of a few MG field, how-
ever Hγ and Hδ would show more splitting than Hβ but
both appear to be relatively sharp in comparison. Hβ
may be affected by TiO features from the M dwarf and
there does appear to be a minor glitch in the blue por-
tion of the spectrum, indication difficulty with SDSS flux
calibration.
5. DISCUSSION
Of the 1253 potential close binary systems in the DR5
catalog, there were 168 systems that we could not mea-
6 Silvestri et al.
sure with our program. These include the :+dM systems
and binaries with non-DA WDs. We were not able to
unambiguously determine if the :+dM systems have a
magnetic or a non–magnetic WD as the blue component
is barely visible in most of the :+dM SDSS spectra. Un-
til we can identify the companion, we can not make any
statement about magnetism in these objects. The :+dM
cases where a blue component is seen in the spectrum
which must be a WD, but too faint even to classify the
type may include (a) cases where the WD is simply very
cool, but also (b) magnetic WDs of suitably warmer ef-
fective temperature but with smaller radii. These need to
be reobserved in the blue with a spectrograph and tele-
scope of large aperture. We made no attempt to measure
the DB WDs because we lack viable magnetic DB WD
models; however, all of the DB spectra matched well to
non–magnetic DB WD models, so we believe it is un-
likely that any of the WDs in these pairs are magnetic.
We could not measure the pairs with DC WDs because
there are no features with which we can detect a magnetic
field and therefore cannot rule out magnetism without
employing polarimetry or other methods of identifying a
magnetic field in these objects.
Of the remaining binary systems, we find only two
that may contain WDs with weak magnetic fields. Our
automatic detection methods are sensitive to magnetic
fields between 3 MG ≤ BWD ≤ 30 MG; field strengths
larger than this are easily identified by visual inspec-
tion. Therefore, there is a significant shortage of close
binary systems that could be the progenitors of the large
Intermediate–Polar and Polar CV populations.
As mentioned in §1, Schmidt et al. (2005b) discuss six
newly identified low accretion rate magnetic binary sys-
tems as being the probable progenitors to magnetic CVs.
The magnetic field strengths of the WDs in these sys-
tems are fairly high, with most around 60 MG. These
objects are clearly pre-Polars and provide an obvious link
between post–common envelope, detached binaries and
Polars. The existence of these objects, however only adds
to the mystery. If observations of these objects are possi-
ble then why have no detached binary systems with large
magnetic field WDs been detected?
Perhaps selection effects are to blame. Schmidt et al.
(2005b) discuss the various selection effects associated
with targeting these pre–Polars with the SDSS. As is
the case with the majority of the close binary systems,
the pre–Polars were targeted by the SDSS QSO target-
ing pipeline (Richards et al. 2002) which accounts for the
narrow range of magnetic field strengths found in these
objects. In the case of significantly lower or higher mag-
netic field strengths, the pre–Polars resemble an ordinary
WD + M dwarf binary in color–color space and are re-
jected by the QSO targeting algorithm. It is possible that
this selection effect accounts for the lack of close binary
magnetic systems targeted by the SDSS as well. Arguing
against this explanation is the large number of detached
close binary systems in our sample, and the fact that the
pre–Polars were observed by the SDSS. It is quite sur-
prising that a detached system with a WD magnetic field
in the range required to detect these pre–Polars has not
been observed, if such objects exist.
Another selection effect discussed by Liebert et al.
(2005) argues that magnetic WDs, on average, are more
massive than non–magnetic WDs; this implies smaller
WD radii and therefore less luminous WDs. Faint, mas-
sive WDs in competition with the flux from an M star
companion might go undetected in an optical survey
because they are hidden by the more luminous, non–
degenerate companion. This would imply an unusually
small mass ratio (q = M2/M1) for the initial binary if
the progenitor of the magnetic white dwarf were mas-
sive (3-8 M⊙). Thus, the magnetics may usually have
been paired with an A-G star. However, the vast major-
ity of polars and intermediate polars with strongly mag-
netic primaries have M dwarf companions. Perhaps they
were whittled down from more massive stars by mass
transfer. The LARPS are selected for spectroscopy be-
cause of their peculiar colors, which arise because of the
isolated cyclotron harmonics. As Schmidt et al. (2005b,
2007) point out, the WDs in LARPS are generally rather
faint (cool) and, in one case, undetected. So the large
mass/small radius selection effect would also apply to
the pre–Polars which have been observed by SDSS.
6. CONCLUSIONS
We present a new sample of close binary systems
through the Data Release Five of the SDSS. This cat-
alog includes more than 1200 WD + M dwarf binary
systems and represents the largest catalog of its kind to
date.
We have fit magnetic DA WD models (see
Schmidt et al. 2003, and references therein) to the 1100
DA WD + M dwarf close binaries in the DR5 sample.
Only two have been found to potentially harbor a mag-
netic DA WD of low (BWD < 10 MG) magnetic field
strength. Neither of these potential magnetic WDs are
convincing cases, though follow–up spectroscopy to im-
prove the S/N or polarimetry on these objects should
be performed to completely rule out the presence of a
magnetic field.
The remaining ∼ 100 close binaries comprised of M
dwarfs with excess blue flux (:+dM) and binaries with
non–DA WDs require other means of detecting mag-
netic fields. Methods that are sensitive to magnetic
fields weaker than 3 MG should also be employed on this
sample to detect possible Intermediate–Polar progenitors
that may have escaped detection with our methods.
Even if future spectroscopic or polarimetric observa-
tions reveal the two DA WD candidates to be magnetic,
their field strengths will likely prove to be quite low. A
sample of two, detached, low magnetic field WD binaries
is not representative of the majority of known magnetic
WDs in CVs nor would it comprise an adequate progen-
itor population for the newly discovered magnetic pre–
Polars described in Schmidt et al. (2005b). The question
of where the progenitors to magnetic CVs are remains
unanswered by the current spectroscopically identified
close binary population.
This work was supported by NSF Grant AST 02–05875
(NMS, SLH), a University of Washington undergraduate
research grant (MPL), NSF grant AST 03–06080 (GDS),
and NSF grant AST 03–07321 (JL).
Funding for the SDSS and SDSS–II has been pro-
vided by the Alfred P. Sloan Foundation, the Partic-
ipating Institutions, the National Science Foundation,
the U.S. Department of Energy, the National Aeronau-
Magnetic WDs in CV Progenitors 7
tics and Space Administration, the Japanese Monbuka-
gakusho, the Max Planck Society, and the Higher Educa-
tion Funding Council for England. The SDSS Web Site
is http://www.sdss.org/.
The SDSS is managed by the Astrophysical Research
Consortium for the Participating Institutions. The Par-
ticipating Institutions are the American Museum of Nat-
ural History, Astrophysical Institute Potsdam, Univer-
sity of Basel, University of Cambridge, Case Western
Reserve University, University of Chicago, Drexel Uni-
versity, Fermilab, the Institute for Advanced Study, the
Japan Participation Group, Johns Hopkins University,
the Joint Institute for Nuclear Astrophysics, the Kavli
Institute for Particle Astrophysics and Cosmology, the
Korean Scientist Group, the Chinese Academy of Sci-
ences (LAMOST), Los Alamos National Laboratory, the
Max–Planck–Institute for Astronomy (MPIA), the Max–
Planck–Institute for Astrophysics (MPA), New Mexico
State University, Ohio State University, University of
Pittsburgh, University of Portsmouth, Princeton Uni-
versity, the United States Naval Observatory, and the
University of Washington.
http://www.sdss.org/
8 Silvestri et al.
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Magnetic WDs in CV Progenitors 9
TABLE 1
The SDSS–I DR5 Catalog of Close Binary Systems.
Identifier Plate FiberID MJD Sp1+Sp2a R.A.b Decl. upsf σu Au gpsf σg Ag rpsf σr Ar ipsf σi Ai zpsf σz Az Refs
c Notesd
(SDSS J) (deg) (deg)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
001029.87+003126.2 0388 545 51793 DZ:+dM 2.62448 00.52396 21.93 0.19 0.14 20.85 0.04 0.10 19.98 0.03 0.08 19.00 0.02 0.06 18.42 0.04 0.04 EDR
001726.63−002451.2 0687 153 52518 DA+dMe 4.36099 −00.41422 19.68 0.04 0.14 19.29 0.03 0.10 19.03 0.02 0.07 18.19 0.02 0.06 17.54 0.03 0.04 R03
001733.59+004030.4 0389 614 51795 DA+dM 4.38996 00.67511 22.10 0.40 0.13 20.79 0.14 0.10 19.59 0.03 0.07 18.17 0.02 0.05 17.39 0.02 0.04 EDR/R03
001749.24−000955.3 0389 112 51795 DA+dMe 4.45519 −00.16539 16.57 0.02 0.13 16.87 0.02 0.10 17.03 0.01 0.07 16.78 0.01 0.05 16.47 0.02 0.04 EDR/R03
002620.41+144409.5 0753 079 52233 DA+dMe 6.58505 14.73597 17.57 0.01 0.27 17.35 0.01 0.20 17.34 0.02 0.15 16.65 0.01 0.11 16.04 0.02 0.08 DR2
Note. — Table 1 is published in its entirety in the electronic edition of the AJ. A portion is shown here for guidance regarding its form and content. ugriz photometry has not been corrected for Galactic extinction.
Sp1: Spectral type of the WD, Sp2: Spectral type of the low–mass dwarf (see Silvestri et al. 2006, for details on Sp determination); e: emission detected visually.
R.A. and Decl. are J2000.0 equinox.
EDR: Stoughton et al. (2002); DR[1,2,3]: Abazajian et al. (2003, 2004, 2005); DR[4,5]: Adelman-McCarthy et al. (2006, 2007); R03: published in Raymond et al. (2003); K04: published in Kleinman et al. (2004); B05: published in van den Besselaar et al. (2005);
Sc05: published in Schmidt et al. (2005a); S06: published in Silvestri et al. (2006); E06: published in Eisenstein et al. (2006); P05: published in Pourbaix et al. (2005); KM: published in Koen & Maxted (2006); SN: published in Schuh & Nagel (2006); da06: R. da Silva
(priv. comm., 2006).
low: potential low gravity (log g < 7) white dwarf.
10 Silvestri et al.
TABLE 2
Two Potential Magnetic White Dwarf Binary Systems.
Identifier Plate Fiber MJD R.A. Decl. B i Sp1+Sp2 u g r i z Release
SDSS J (deg) (deg) (MG) (deg)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)
082828.18+471737.9 0549 338 51981 127.11742 +47.29387 8 90 DA+dM 20.41 20.35 20.33 19.58 19.02 DR1
125250.03−020608.1 0338 343 51694 193.20846 −02.10227 3 90 DA+dM 19.25 19.12 18.89 18.31 17.82 DR1
