WHITE DWARF DETECTION IN THE ALHAMBRA SURVEY
S. Catalán, J. Isern, E. García-Berro, S. Torres, Alhambra Team
Institut d’Estudis Espacials de Catalunya, (IEEC/CSIC/UPC), Barcelona, SPAIN
A new high visual depth survey is presented. New cool white dwarfs will eventually be discovered, which will undoubtely increase the statistical significance of the white dwarf luminosity
function. Moreover, by increasing the sample of known white dwarfs, we will be able to determine which ones are members of a binary system and which of those binary systems are detached
and composed by a main sequence star and a white dwarf. Hence, comparison of their corresponding ages will provide tight constrainsts on the evolutionary models.
THE ALHAMBRA SURVEY
Figure 1. Fields chosen by the ALHAMBRA Team.
Figure 2. Transmission of each filter versus wavelength
The ALHAMBRA (Advanced Large, Homogeneous Area Medium
Band Redshift Astronomical) survey will observe eight 1-squared
degree fields, four in each galactic hemisphere to allow continuous
observations throughout the year.
Fig.1 shows the location of the proposed fields. Some of them have
been chosen intentionally to overlap with other publicly available
surveys for the sole purpose of verifying calibration methods and
comparing results. The other fields have been selected close to the
celestial equator to allow future follow-up in both hemispheres.
Figure 3. Limiting AB magnitudes versus wavelength for different filters.
The ALHAMBRA survey is a general-purpose survey whose primary aim is to provide a large-area (8 squared
degrees), magnitude-limited photometric catalogue in 20+3 bands, which will include reliable and precise photometric
redshifts for ~106 galaxies and AGNs. The system of filters (fig. 2) is composed by 20 marginally overlapping, mediumband, top-hat filters in order to provide a complete, contiguous spectral coverage in the optical wavelength range (35009700Å), and by filters JKH which will take images in the near-IR bands.
In fig. 3, the limiting AB magnitudes are expressed as a function of wavelength for different filters in the optical
domain. The goal of this survey is to reach, in the optical range, a constant flux, AB=25 at S/N=5, in all the filters
covering from 3500 to 8300Å, and from AB=24.7 to 23.4 for the remaining filters. The exposure time necessary to
reach the quoted limit has been computed for each filter and it is indicated in ksecs in each of the bins.
COLOUR-COLOUR DIAGRAM
Once the results of the ALHAMBRA
survey become available, it will be
necessary to place those objects of
interest in a colour-colour diagram
(fig. 5), which relates photometric
values (magnitudes and colours) with
physical
properties
(effective
temperature and surface gravity).
Figure 5 shows an example of a wide
grid which has been plotted having
considered the absolute magnitudes
M1, M5 and M11. These magnitudes
have been obtained taking into
account the brigthness, b, collected
within the filters 1, 5 and 11 from the
photometric system of this survey.
A set of different temperatures (7000 to
60000K) and surface gravities (107-109
cm/s2) DA white dwarfs theoretical
spectra from the Koester database has
been used to compute AB magnitudes
for each spectra (red line).
An example is shown in fig. 4, where a
typical DA WD spectra can be seen.
The Hydrogen absorption lines: Hα
(6563Å), Hβ (4861Å), Hγ (4340Å) are
quite noticeable as are some other
transitions near 4000Å.
M AB = −2.5 log(b) − 48.60
Figure 4. AB magnitudes and theoretical spectra versus
wavelength for a DA WD with Teff=14000K and log(g)=8.0.
b=
Figure 5. Colour-colour diagram for a set of DA white dwarfs
λ2
∫ F ( λ ) Aλ ( z ) R ( λ ) d λ
λ1
DISCUSSION
WHITE DWARF DETECTION
Assuming a standard intial mass function, a constant star
formation rate and updated models of white dwarf cooling,
the white dwarf luminosity function can be computed, and
from it, the expected number of white dwarfs can be
derived for each field.
A total number of 300 WD is expected to be detected in the
ALHAMBRA survey, 38 in each of the fields shown in
figure 1.
In this table it is shown the number of white dwarfs, in the
galactic disk and in the galactic halo, that are expected to
be detected in each of the fields proposed by the
ALHAMBRA survey.
Figure 6. Transmission of some of the Alhambra filters and
the V filter versus wavelength.
The V filter can be expressed as a combination of those Alhambra filters
which fall within its wavelength range (fig. 6):
TV = 0.64T1 + 1.79T2 + 1.41T3 + 0.78T4 + 0.24T5 - 0.13T6
where TV and Ti are the transmissions of filter V and of each filter,
respectively. The visual limiting magnitude computed in this way turns out
to be 25.5 and, hence, the ALHAMBRA Survey will provide a handful of
faint WDs.
FIELD
CO SM O S
H D F-N
G RO TH
ELA IS-N 1
A LH A M BRA -1
A LH A M BRA -2
A LH A M BRA -3
A LH A M BRA -5
D ISK
36
31
32
37
51
33
33
30
H A LO
2
1
1
2
3
2
1
2
COMPARISON WITH OTHER
SURVEYS
A comparison of the main characteristics of the
ALHAMBRA Survey and those of some publicly available
surveys is shown in the next table:
SURVEY
COSMOS
HDF-N
SDSS
NOAO
COMBO-17
ALHAMBRA
IAB
limit
27.0
28.0
21.3
26.0
24.0
25
AREA Spectral
Resolution
(º2)
Range
2
gI
2.5
0.001
UVI
4
8000
Ugriz
6
18
BRIJHK
4
1
3650-9140 Å
25
8
3500-22000Å
25
The AHAMBRA Survey will provide observations over a
large area, as well as very high resolution and deep visual
detection. This is the most important aspect that
distinguishes this survey from the rest.
REFERENCES:
Isern, J., García-Berro, E., Hernanz, M., Mochkovitch, R., and Torres, S.
1998, ApJ, 503, 239
Koester, D., private comunication to Jordi Isern.