Effective temperatures of 15 DA white dwarfs hotter than 20,000 K were derived from low-dispersion far ultraviolet spectra obtained with IUE. The analysis was carried out by comparing the observed far ultraviolet fluxes with model fluxes scaled to the V-band flux. Accurate calibration of the IUE spectra is critical for this analysis. Observations at all epochs were corrected to the 1980 IUE calibration using the time-dependent corrections of Bohlin (1988). Taking advantage of the smooth and well-defined continuum fluxes provided by DA white dwarfs, seven white dwarfs for which accurate, independent temperature determinations were made from line profile analyses were used to improve the accuracy of the IUE flux calibration. The correction to the original calibration is as great as 20 percent in individual 5 A wavelength bins, while the average over the IUE wavelength range is 5 percent. The final calibration correction and the temperatures for the hot white dwarfs are presented

Basri, Gibor

Bowyer, Stuart

Finley, David

English

NASA Technical Reports Server

267
PR_]DING PAC-E __LA?-_'_ U_? _fLMED N89 - 10 8 4
TEMPERATURE DETERMINATIONS OF HOT DA WHITE DWARFS USING IUE CONTINUUM FLUXES
David Finley Olbor Bash Stuart Bowyer
Space Sciences Laboratory Astronomy Department
University of Cahfornia, Berkeley, California USA
Astronomy Department and
Space Sciences Laboratory
ABSTRACT
Effective temperatures of 15 DA white dwarfs hotter than 20,000
K were derived from low-dispersion far ultraviolet spectra which
were obtained with the International Ultraviolet Explorer (IUE).
The analysis was carried out by comparing the observed far ultra-
violet fluxes with model fluxes scaled to the V-band flux. Accu-
rate calibration of the IUE spectra is critical for this analysis We
first corrected observations at all epochs to the 1980 IUE calibra-
tion using the time-dependent corrections of Bohhn (Refs. 1,2).
Taking advantage of the smooth and well-defined continuum
fluxes provided by DA white dwarfs, we then used seven white
dwarfs for which accurate, independent temperature determinauons
have been made from line profile analyses to improve the accuracy
of the IUE flux calibration The correction to the onglnal calibra-
tion is as great as 20% In mdlvlduat 5-/_ wavelength bins, while
the average over the IUE wavelength range is 5%. We present
both the final calibration correction and the new accurate tempera-
tures for the hot white dwarfs
Keywords' White Dwarfs, IUE, Ultraviolet, Spectrophotometry
I. INTRODUCTION
Accurate temperature determmanons for hot DA white dwarfs are
necessary for several reasons. Temperatures are needed for denv-
ing the luminosity function of DA's; the luminosity function then
serves to check calculataons of coohng rates for these stars. Trace
element abundances m DA's result from temperature-dependent
processes. Successful confrontation of observauonal abundance
determinations with theory requires that the effective temperatures
be known with sufficient accuracy. Also, the upper temperature
limit for DA's needs to be determined with precision, because this
limit will help constrain post mare sequence evolunonary calcula-
tions. Addmonally, upcoming extreme ultraviolet (EUV) pho-
tometric survey missions (Refs. 3,4) are likely to discover hun-
dreds of very hot DA white dwarfs (Ref. 5). Non-EUV measure-
ments will be reqmred to make the accurate temperature determi-
nations necessary for interpretation of the EUV photometric data
for these stars (Ref 5).
Temperatures of DA white dwarfs have usually been based on
optical photometric measurements However, the optical colors
become poor temperature indicators for stars hotter than -25,000
K. Given that FUV continuum fluxes should serve as a more sen-
sitive temperature indicator, an IUE observing program was ini-
tiated that specilically targeted DA white dwarfs hotter than
20,000 K (Ref. 6). It was &scovered m the course of analyzing
the data from that program that the sensluvlty degradation of the
IUE cameras gave rise to s_gnlficant errors in the temperatures
inferred from the IUE fluxes, compared with optical photometric
temperatures. Similar problems had been noted by 1UE observers
of other object types (Ref. 7). Consequently, the data analysts
presented in Ref. 6 was carried out by making a correction to the
IUE fluxes which achieved consistency between the FUV-denved
temperatures and the optical photometric temperatures. Because
of the ume dependence of the sensltiV W degradation, this correc-
lion consisted of using a few well-observed stars from each -1-
year observing epoch to obtain the necessary sensmvity adjust-
ment for that epoch (Ref. 6) Subsequently, efforts were under-
taken by other workers to obtain time-dependent corrections which
could be used to correct for the rime and wavelength-dependent
sens,tlvlty degradation of IUE; these correcuons are now available
(Refs. 1, 2, 8) The stars from the observing program discussed in
Ref 6, as well as a numbel of archive spectra, were analyzed on
the basis of these time-dependent corrections This paper presents
the results of these analyses.
2 DATA ANALYSIS AND RESULTS
The observations were obtained with the IUE's short wavelength
primary (SWP) and long wavelength redundant (LWR) cameras
operating m the low-dispersion mode Stars which were
sufticmntly bright were observed with both cameras Fainter stars
reqmrlng long exposures were observed only with the SWP
The IUE spectra were reduced with standard IUE processing tech-
niques. For the analysis, the SWP spectra were truncated at the
short wavelength end at 1320 _ to avoid the red wing of Lyman
ct. The SWP and LWR spectra were cut off at 1940 /_, and the
LWR data longward of 3100 /_ were omitted because of the
unreliability of the fluxes beyond that point. The IUE fluxes were
then placed on the original IUE flux scale according to the
prescription of Bohlin (Refs. 1, 2). This process only corrects for
the variation with time of the IUE sensitivity, and _s independent
of the absolute flux calibration of the IUE instruments. Bohhn's
time-dependent correction data set consists of discrete values
which are averaged over 1-year time intervals and 5-/_ wavelength
intervals Each data point m the observed spectra was modified
with a correction value that is linearly interpolated in time but is
based on the values for the nearest wavelength. Bohlin estimates
that the correct,ons given by this procedure are accurate to -1%.
This procedure was applied to all the spectra. However, the spec-
tra for WD0004+330, WD0644+375, and WD2309+105
(SWP7747 arid LWR6452) were reduced with the old IUE spectral
extraction scheme, which was used until November 1980; hence,
Bohhn's procedure will not give optimal results for these spectra.
A Decade of UV Astronomy wah IUE, Pro_ Celebratory Srmposttml, GSFC, Greenbelt, USA, 12-15 April 198& ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001363 2020-03-20T06:09:02+00:00Z
268 D FINLEY. G BASRI & S BOWYER
Normahzataon of the IUE data was accomphshed by rallomg the
FUV fluxes to the V-band flux, given by
f(5490_) = 3.61x10 9/10°4mv
Determination of the effective temperatures was then accomplished
by companng the observed f(_.)/f(5490_) to that predicted by
model atmosphere fluxes for a given effective temperature. The
model atmosphere code used was that developed by Basri (Ref. 9).
This code, based on Auer's complete hnearization method, treats
the atmosphere as being plane-parallel and in LTE. The contin-
uum fluxes produced by this code have been checked against
model fluxes published by Shipman and by Wesemael (Refs. 10,
11), and have been found to agree within 1% from the FUV
through the visible, for effective temperatures greater than 25,000
K. The models allow the presence of trace helium; however, the
helium abundance within the DA range of <10 -2 has an effect on
FUV/visible flux ratios only at the <3% level. Therefore, a nomi-
nal helium abundance of 10 -4 was used in the model calculauons
The effect of surface gravity on FUV/visible flux rataos is less
than 1% over the range 107 to 109; hence a value of 10 s was
used. Temperature determinataons were made by comparing the
data with a range of models and interpolating to find the best fit
model temperature. The best fit temperature was taken to be that
for which
2,-25,og ..... : 0
Evaluation of the residuals of the fits of the models to the time-
corrected data revealed systematac variations as a function of
wavelength These variations were of the order of +15%. The
wavelength dependence of the residuals was consistent from spec-
trum to spectrum Furthermore, comparison of the temperatures
obtained from the corrected IUE fluxes with temperatures obtained
from hydrogen line profiles (Refs. 12, 13) indicated that the IUE
fluxes were low by 5% on the average.
Several factors might account for the observed discrepancy. For
instance, the model physics may be wrong. The stellar atmo-
sphere composition may not be pure hydrogen, or the stars may be
reddened. Also, the line profile temperatures may be in error.
None of these can explain the wavelength-dependent irregularities
in the fluxes, those must result from IUE calibration errors. The
uncertainties in the model physics affect the average flux level at
the <2% level (Ref. 10). Reddening cannot account for the
apparently low FUV fluxes, because these are nearby (<100 pc)
white dwarfs for which the neutral hydrogen columns are much
less than 102°/cm 2. Unless the exunctlon is highly anomalous, a
typical neutral hydrogen column of 3×1019 would give
EB_ v = 0.m005, resulting in an average extinction over the 1UE
wavelength range of only 1 6%. The temperatures denved from
line profiles are subject to small uncertainties due to the statistacal
errors in the data; Holberg (Ref. 12) quotes formal errors of only
+3,000 K at 55,000 K. Systemaac errors due to instrumental cali-
bration errors over the small wavelength ranges of hydrogen lines
are unlikely to significantly affect temperature determinations.
Therefore, aside from the possibility of the existence of errors in
the model line profiles, the effective temperatures for DA wNte
dwarfs that are derived from hydrogen line profiles are the most
accurate available. Consequently, the observed systematic varia-
uons between the IUE fluxes and the model fluxes are most likely
attributable to errors in the IUE calibration.
In order to reconcile FUV continuum temperatures with the line
profile temperatures, spectra from seven white dwarfs were used
to obtain a flux correction to the IUE data. The stars used were
WD 0050-332, 0501+527, 0549+158, 1254+223, 1620-391,
2111+498, and 2309+105 (1982 observation only). Model fluxes
were generated for each of these stars using the effective tempera-
tures given by Holberg (Ref. 12). Ratios of the observed
FUV/vlslble flux ratios to the values predicted by the models were
calculated for all seven spectra These ratios were then avenigcd
together over the same 5-_ wavelength bins used by Bohhn The
resultant flux correction thus consists of the values by which
time-corrected fluxes must be divided The flux correction Is
shown in Figure 1.
2
o
cu 1.4
o
o 1.2
I"4
O
1
(D
N
.6
-I I I i i i i I i i i i I r , i i I 1-
1500 2000 2500 3000
Wavelength (_.)
Figure 1. Flux correction to 1UE original epoch calibranon,
binned in 5-_ intervals Spectra to which time-dependent
correction has already been applied are to be divided by the
flux correcnon values.
After application of both the time-dependent and flux corrections
to the spectra, the effective temperatures were recomputed The
complete listing of the results obtained is given m Table 1. The
absence of any sigmficant variation with temperature of the FUV
continuum temperatures compared to the line profile temperatures
indicates that any temperature dependent effects which may affect
the modeling of either the line profiles or the continuum fluxes are
not significant.
Temperatures are given both for spectra which were time- and
flux-corrected (Method 1) and for spectra subjected to individual
epoch corrections (Method 2) calculated as per Ref. 6. In the
current instance, the individual epoch corrections were based on
line profile temperatures, rather than on optical photometric tem-
peratures, as had been done in Ref. 6. The epochs for which the
corrections were obtained, and the stars used, were' 1980.0
(WD2309+105), 1981.4 (WD0501+527, 1254+223, 2111+498),
and 1982.6 (WD0050-332, 0549+158, 1620-391, and 2309+105).
It is seen that the two methods are equivalent, giving effective
temperatures which differ only trivially. Note also that, although
the time-dependent correction cannot be strictly applicable to spec-
tra reduced prior to November 1980, the effecuve temperatures
that are derived using that scheme are very close to the tempera-
tures obtained from the individual epoch corrections.
The errors shown for the IUE continuum measurements are the
combined errors arising from uncertainties in the V magnitude, the
spectral-signal-to-noise ratio, the values of stellar parameters other
than effective temperature, small reddening effects, and errors in
the flux correction. In addition, the flux correction error included
the published uncertainties in the effective temperatures used for
obtaining the flux correction. (Uncertainties in the model fluxes
have no effect; the models are used to obtain the flux correctaons
and to fit the corrected data. Hence, any systematic model-
dependent effects would cancel out ) The magnitude of the flux
correction error averaged over the IUE wavelength range (for the
correction derived from all seven stars) was only +1.3% (1 _),
"I'I-MPI:RATURE I)ETI_RMINATIONS ()F H(YI" DA WHITE DWARFS 269
Table 1 Temperatures of Slars Derived from Different Measures
[
WD Name
IUE Continua
Method 1_' Method 2 b
Teff c Error a Tef f Error
Line Profiles
ErrorTef f Ref.
0004+330 42.58 e +346 42.84 +560 ...... 56.5 _ 197
-2 83 -4 13 -,07
0050-432 34.15 +1 64 34.68 +l 84 36.85 +1.39 HWB 33.51 -_, 57
-1 37 -1 52 -, 25
40.33 4697
0346-011 39.10 +22, 38.33 +222 47.5 :t:2 5 K 72/_4?
-J 90 -189 31.44
-1 52
0501+527 66.37 +876 63.82 +869 62 25 ±3.52 HWB 71.7 +L6o
-7 13 -7 05 -11 4
0549+158
0644+375
0651-020
1031-114
1033+464
1254+223
1403-077
34.03 +, 62 34.32 +176
-1 35 -I 46
21 76 e +029 21 84 +o43
_329 -041
35.50 +247 35.84 +2 65
-I 96 -2 08
25.67 +936 25,81 +039
+034 +037
28.38 +48, ....
-230
40.67 -_265 40.20 +277
-2 22 -2 29
45.98 +818 ....
-5 78
31 76 +085 31,40 +088
4) 76 +078
33.30 5:0.83 HWB
42 375 +1.48 ftWB
42 -t_:2 K
1615-154
Optical
Photomelry
Tot f Error
37 77 -_353
-2 56
22.72 m 34
-0 33
36 73 4-_76O6
25,03 040
-(J d8
"27 2"
41 91 *_ 42
-2 77
41 04 +737
-4 59
30 83 +o 68
+0 60
1620-391 24.83 +039 24.92 -:o41 24 50 :L-_3.14 HWB 24 37 +047
_339 -041 +046
2111+498 36.125 :k094 HWB37.36 +232 36.76 +234
-I 91 -190
5030 / +42_ 5203 f +491
-3 57 -4 08
52 69 e'g +474 53 54 g +723
-3 98 -5 63
53.60 +2 94 ttWB2309+105
37.15 +257
-2 O1
65 2 +m 1
~7 7
%_Icthod 1 Involved using time and llux corrections to IUE spectra as per section 5 3 6 2
bMethod 2 involved using individual epoch correctam_ a,, per sectmn 5 3 6 1
q'emperatures are given In 10 3 K
aErrors are lo uncertainties
q'emperatures obtained using Method | orJ these ob_;ervatloBs arc not strictly accurate See text
"IUE temperatures were oblamed from 1982/142 observahons
8IUE temperatures were obtained from 19791355 ob_,ervatlons
Relerences llWB = Holberg, Wesemacl, and Basdc (Rcf 12) K - Kahn et al (Rcl. 13)
excepting possible flux errors due to uncertainues in the model
physics Shipman has estimated, however, that the model uncer-
tainties are of the order of 2% or less (Ref. 10) The errors in the
temperatures of the ind,wdual objects were dommamd by observa-
tional uncertamnes m the V magnttudes
Also listed in Table 1 are the pubhshed line profile temperatures
and temperatures calculated from published opUcal photometry.
The opncal photometric temperatures are taken from Ref. 5. The
temperatures obtained from the different measures are consistent m
all cases except that of WD0346-011. For this star, the three
&fferent measures g,ve three different temperature.s, indicating that
the atmosphere of this star might not be homogeneous.
3. CONCLUSIONS
The original epoch cahbration of IUE produces tluxe_ for DA
white dwarfs whmb systemancally vary with respect to fluxes
predmted by model atmospheres. The vananons w,th wavelength
are of the order of _+15% Based on predmted fluxes using
effecnve temperatures of DA whne dwarfs derived from fitting
hydrogen hne profiles, the IUE flux levels after correcnon for the
ume-dependent sensitivity degradanon are 5% low (averaged over
the range 1320 to 3100 ,_). DA white dwarfs may be used to
derive a flux correction which can be applied to achieve an abso-
lute overall accuracy for IUE spectra of the order of 5:2%.
270 D FINLEY. G BASRI & S BOWYER
4. ACKNOWLEDGMENTS
This research was funded by GSFC Contract #NAS5-29298.
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Temperature determinations of hot DA white dwarfs using IUE continuum fluxes

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