4 Hu, Wu & Wu: DA white dwarfs in the SDSS
0.3 0.5 0.7 0.9 1.1
0.5
0.7
0.9
1.1
M
other
M
spec
Fig. 2. A comparison between the WD mass derived from
the evolutionary model and those determined by other
methods without us ing a theoretical M-R relation. Both
axes are in unit of M
⊙
. All po ints in the diagram are
listed in Table 1. Triangles repre sent the WDs with mass
determined by the triangle parallaxes and surface grav-
ity. Circles represent WDs with mass determined by the
gravitational redshifts. Squares repres ent WDs with mass
derived from the orbital parameters. Crosses represents
WDs cooler than 12000K. The dashed line repre sents
M
spec
= M
other
.
are obviously larger. This is beca us e these cooler WDs are
likely to be convective. BSL have convincingly proved that
the convection effect leads to significant amounts of helium
(which is invisible in the spectra) entering the atmosphere,
producing higher pressure which would substantially af-
fect the s pectral line profiles. The total effect on the spec-
tral line is indistinguishable from the increased surface
gravity. In other words, a low-tempera tur e DA WD with
large surfac e gravity might actually be a helium-rich star
with lower surface gravity (and correspondingly with lower
mass). So the scatter in the ma sses estimated with differ-
ent methods for cooler WDs possibly has less to do with
the evolutionary model tha t we adopted but is mainly due
to the techniques o f analyzing the spectra l lines. For this
reason, we remove these WDs from our statistical a naly-
ses.
There are still some high-temperature WDs for which
the different mass estimates do not match very well. A few
factors can contribute to this discrepancy, such as the tech-
niques of fitting the spectral lines, the uncertainties of the
observational parameters. etc. One of the most important
factors is that there seems to be no appr opriate evolution-
ary model fo r these high-tempera tur e WDs. For example,
G238-4 4, GD140, EG50, and EG2 1 have re latively higher
sp e c troscopic mass compared with the mass derived from
other methods. If we apply a thin hydrogen layer model
(q(H) = −1, q(He) = −4) or a metal core (like F e core)
for these four WDs, their spe c troscopic mass will be lower
by 0 .04∼0.06 M
⊙
, and thus the mass comparison of these
four WDs would be better. Moreover, the presence of he-
lium in the atmosphere would also significantly influence
the mass estimate. Boudreault & Bergero n (2005) gave
a detailed discussion of this effect. They calculated the
masses by using the models of Fo ntaine et al. (2001) and
assuming a mixed composition in the atmosphere with
M(He)/M(H) = 1 rather than a pure hydrogen atmo-
sphere, and obtained similar results that the mea n M
spec
of WDs in their sample will be lower by 0.2 M
⊙
. Thus, if
we adjust the thickness of the envelope, the composition
of the atmosphere and the atom in the core, more than
half of the WDs in Fig. 2 will have their M
spec
equal to the
mass derived by the o ther method. Therefore, we may find
the most appro priate e volutionary model for each WD by
matching two kinds of mass estimates, and then the dis-
crepancy in Fig. 2 would be alleviated.
However, for most DA WDs from SDSS DR1 in our
sample, we do not have paralla x or gravitational redshift
data to derive a second mass estimate and do not have fur-
ther information about their interna l structure and atmo-
spheric composition. So we will just ass ume a theoretically
appropriate model for our samples. From Fig . 2, we find
that the comparison results are satisfactory in general, ig-
noring the low-temperature WDs. We then conclude that
the assumptions of evolutionary models we adopted are
generally reliable.
After testing the applicability of the model of Panei et
al. (2000), we use it to c alculate the masses of SDSS DA
WDs in our sample. From the T
eff
-log g diagram shown
in Fig. 1, we can see that using the two parameters T
eff
and log g we can determine the mass of the WDs (the the-
oretical lines can be interp olated to cover the area that
the lines do not cover in the figure). The other parame-
ters of the WDs also can be calc ulated based on the mass
estimation.
Using the methods described above, we can calcu-
late the radius R, luminosity L and bolometric mag-
nitude M
bol
for WDs. Similar to the determination of
mass, the cooling age of a WD can be der ived by in-
terpolating the grids of the evolutionary models, thus
log Age = log Age(T
eff
, log g). The bolometric cor rec-
tion (B.C.) in the g band can be obtained by using the
model atmospheres of Bergeron et al. (1995b). B.C . is de-
rived through interpolating the T
eff
and log g into the
grid of the model atmosphere in Bergeron et al. (1995b)
for the ugriz system, B.C. = B.C.
Bergeron
(T
eff
, log g).
The distance r (in p c) of the star can be derived from the
B.C. and the relations hip between the absolute magnitude
and visual magnitude in the g band: M
bol
= M
g
+ B.C.
g
,
M
g
= g − 5 − 5 log r − A
g
, where A
g
is the extinction in
the g band which is provided by the SDSS.
To compare our re sults with other pr e vious work, we
also derived the absolute magnitude of WDs in the V band
which were commonly used in previous studies. Using the
results of B e rgeron et al. (1 995b), we ca n easily c onvert
剩余15页未读，继续阅读
weixin_38677648
- 粉丝: 5
- 资源: 886
我的内容管理
展开
最新资源
- AirKiss技术详解：无线传递信息与智能家居连接
- Hibernate主键生成策略详解
- 操作系统实验：位示图法管理磁盘空闲空间
- JSON详解：数据交换的主流格式
- Win7安装Ubuntu双系统详细指南
- FPGA内部结构与工作原理探索
- 信用评分模型解析：WOE、IV与ROC
- 使用LVS+Keepalived构建高可用负载均衡集群
- 微信小程序驱动餐饮与服装业创新转型：便捷管理与低成本优势
- 机器学习入门指南：从基础到进阶
- 解决Win7 IIS配置错误500.22与0x80070032
- SQL-DFS：优化HDFS小文件存储的解决方案
- Hadoop、Hbase、Spark环境部署与主机配置详解
- Kisso：加密会话Cookie实现的单点登录SSO
- OpenCV读取与拼接多幅图像教程
- QT实战：轻松生成与解析JSON数据
资源上传下载、课程学习等过程中有任何疑问或建议，欢迎提出宝贵意见哦~我们会及时处理！
点击此处反馈
