[Message Prev][Message Next][Thread Prev][Thread Next][Message Index][Thread Index]

[vsnet 55] Bright BVRI Standard Stars

Folks,
     From ASU nova-net and VSnet messages concerning recent novae and 
supernovae, it is clear that many amateur CCD observers continue taking data
either without filters or with widely varying types of nominally "standard"
broadband filters.  If such data is going to be of any use, there really 
needs to be a filter in the light path, and data taken with filters need to be
better calibrated.  Otherwise we'll continue to get inconsistent data from 
different sources as happened with the M81 supernova.
     By way of encouragement, listed below is a collection of 42 bright stars
to serve as standards to determine magnitude zero-points and color terms.  
Since most observers are using small telescopes, and evidently have been using
fairly bright stars to calibrate their data, the list includes only stars 
brighter than V = 8.0, and most are brighter than V = 7.5.  (The brightest has
V = 5.4.)  This makes them easy to find, and in general each star is much the
brightest one in the field, so that identification is unambiguous.
     I first selected stars brighter than V = 7.5 from the main 1983 Landolt 
paper (Landolt 1983a), avoiding wide double stars (difficult to do aperture
photometry on) and those having large uncertainty values, indicative of 
variability.  A few stars were drawn from the unjustly neglected "instrument 
stability" paper (Landolt 1983b).  These were supplemented by brighter stars 
from Taylor's (1986) summary of VRI colors for equatorial standards.  In this
paper, VRI photometry from many sources (e.g. Moffett & Barnes 1979, Crawford,
Golson, & Landolt 1971) was transformed to the Cousins system.  Because bright
stars are relatively scarce in these lists, this initial set had several gaps
in RA coverage.  I thus added selected stars from Landolt (1983a) and Taylor 
between V = 7.5 and 8.0 to fill in both in RA and in color.  The final set 
contains mostly fairly blue and red stars, with only a few of intermediate 
color.  Thus they can be used in red-blue pairs as "extinction" stars (e.g. 
HD 30544 and 30545) as well as for determining rough color transformations.
More exact color transformations should be done with stars throughout the 
color range.
     The table is largely self-explanatory.  I have used HD numbers in
preference to all other designations.  The star-name list is repeated at the
bottom with notes added, including Selected Area and HR numbers, etc.  The 
J2000 positions are from the PPM catalogue, and are given to 1s/0'.1 
precision, sufficient to center a star in the typical CCD frame.  Next come 
the V magnitudes and BVRI colors.  The VRI colors are on the Cousins system.
The spectral types were drawn from wide variety of sources (searched for
using the SIMBAD database).  The final column shows the source of the numbers;
the codes are explained at the bottom of the table.  There is a slight bias 
toward the northern hemisphere where I have added a few stars outside the 
equatorial zone.  Southern observers are already well-served by the excellent
Cousins E-region standards at -45 Dec, so this should not be an inconvenience.
Some of the RI colors from source L83b are based on only one or two 
observations; I have rounded these to two decimals.  (The L83b stars, although
not intended as standards, were observed on the same nights as the main L83a 
standards list and include about as many observations per star.)  The rest of
the values are based on many observations, and should be reliable to within
+/- 0.01 or better.
     Some other general comments:
     1)  For many of the Landolt equatorial standards, "alternative" data is
published by Menzies et al. 1991, based on observations from the South 
Africa Astronomical Observatory.  These results were carefully tied to the 
Cousins E-region standards and have small internal errors.  There are clear 
systematic differences between Landolt and the SAAO group arising from 
instrumental effects, but for the purposes of most amateur observing (i.e. 
from relatively poor photometric sites), the differences are "down in the 
noise".  It is worth mentioning that the trends as a function of RA in V found
by Menzies et al. appear to lie in the SAAO data, not in Landolt's (see the 
mea culpa in Cousins & Menzies 1993).  
     2)  The UBVRI photometric standards that appear in the annual 
"Astronomical Almanac" published jointly by the USNO and RGO are based on the
Johnson et al. 1966 publication containing an all-sky survey of bright
stars.  Although based on measurements using (among others) the original UBV
filters and 1P21 photomultiplier---and hence in effect defining the system,
the data there are noisy by contemporary standards, with rms scatter in the
0.03 range (Landolt's data have internal precision of ~0.005 mag.).  More 
importantly, the VRI colors are on the older Johnson system, which has now 
fallen into disuse as a result of the better-defined and fainter 1983 Landolt 
standards, which are faint enough to observe with large telescopes from both 
hemispheres.  Obviously, these data should be avoided; the Almanac list is 
also in serious need of revision.  It is possible to transform data between 
the Johnson and Cousins RI systems, at least for stars of ordinary color.
Taylor 1986 contains the the excruciating details for photomultiplier systems.
The problems for CCD data taken with rectangular passbands are outlined in a
series of publications by Bessell (see Bessell 1990 for a list, plus UBVRI 
filter prescriptions for CCDs, but also Lee 1994 for cheaper alternatives.)
     3)  Much of the data in the Johnson et al. paper was summarized in a
famous article in the July 1965 issue of "Sky & Telescope" (Iriarte et al. 
1965).  These data were also once frequently cited by professional astronomers
as having been used as standards, even though the article states explicitly 
that they are not: "It should be remembered that the new Catalina-Tonantzintla
observations do not define the UBV system."
     4)  Because your results will have an uncanny habit of working themselves
into the literature, it is important when you send out results over the Net to
include "recovered" values for your standard stars together with results for
the target of interest.  For newly-discovered events, and even most field
variables, there will rarely be previously-observed comparison stars close 
enough to use in a strictly differential mode.  Thus, for many observers the 
most practical approach is to have previously determined color coefficients 
for a system using a large number of stars observed over several clear nights.
Then assuming these values as constants, two or three stars can be used on any
given night to set zero-points for a new "unknown" field.  This is the 
procedure used by the Gilmore-Kilmartin team in New Zealand (with a single-
channel photometer), and Robert Mutel and students in Iowa (using a CCD on a 
small refractor), whose reports appear frequently here.  This avoids having to
establish instrumental coefficients each night, which requires two or three 
dozen stars, and simply can't be done regularly from most sites.  Include the
actual "observed" values for those few nightly zero-point stars with your 
reports on variables.  
     5)  Even if you use no filter, you should try to determine the color term
of your system by using the red and blue stars in the list.  Probably the best
method would be to adjust your "wide-open" CCD magnitudes to the R scale as a 
function of V-R or R-I.  Adjusting to V as a function of V-R or V-I might work
as well for some CCDs.  Of course, R = V + (V-R), and V-I = (V-R) + (R-I).
Meanwhile, work on getting some filters for your system!
     6)  Finally, it should be noted once again that in principle there is no
way to transform data for emission-line objects like novae and supernovae to
any standard photometric scale based on ordinary stars.  The spectra are 
simply too dissimilar to avoid systematic errors between different 
filter/detector combinations.  The problem is most acute toward the blue, but
workable with broadband filters in the red---as long as each system is well 
calibrated.

     Alright, gang, collect them photons!  Go, go, gophers!
\Brian

==========

Bright Equatorial Standards

Name         RA (2000) Dec       V      B-V     V-R     R-I    MK       source
HD   315   0 07 44   -2 32.9   6.440  -0.145  -0.037  -0.064   B8IIISi  L83a
HD  7615   1 16 28  +23 35.4   6.693   0.047   0.06   -0.01    A0       L83b
HD 18145   2 54 47   -0 02.9   6.528   1.048   0.529   0.497   G8II     T86
HD 18175   2 55 14   +0 26.2   7.033   1.129   0.572   0.526   K0II     T86
HD 18369   2 57 10   +0 26.9   6.628   0.327   0.193   0.191   F0IV     T86
HD 30544   4 48 39   +3 39.0   7.316  -0.062  -0.016  -0.016   B9       T86
HD 30545   4 48 45   +3 35.3   6.031   1.207   0.598   0.565   K1III    T86
HD 31331   4 54 51   +0 28.0   5.992  -0.128  -0.046  -0.055   B5V      T86
HD 37334   5 37 37   -4 56.0   7.150  -0.160  -0.066  -0.085   B1.5V    L83b
HD 37981   5 42 58  +14 10.7   6.731   1.096   0.579   0.511   K1IV     L83b
HD 40210   5 57 25   +0 01.6   6.905  -0.005   0.012   0.014   A0V      T86
HD 50167   6 52 04   +1 15.1   7.861   1.535   0.826   0.757   K5       L83a
HD 65079   7 57 04   +2 57.0   7.832  -0.182  -0.055  -0.075   B2Vne    L83a
HD 75012   8 47 35   +0 04.7   7.815   0.088   0.047   0.059   B9       T86
HD 85990   9 55 35   -1 07.6   7.997   1.108   0.575   0.515   K0III    L83a
HD 86135   9 56 39   -0 27.7   7.835   1.485   0.795   0.728   K5       L83a
HD 94864  10 57 08   -0 18.7   6.877   0.421   0.250   0.243   F5       T86
HD 97991  11 16 12   -3 28.3   7.391  -0.227  -0.102  -0.140   B1V      T86
HD118330  13 36 15   -0 55.9   7.062   0.528   0.313   0.311   F8       L83a
HD129956  14 45 30   +0 43.0   5.685  -0.022  -0.010  -0.003   B9.5V    T86
HD139137  15 36 34   -0 33.7   6.509   0.725   0.431   0.412   G8III+A5 T86
HD139308  15 37 29   -0 53.1   7.779   1.275   0.663   0.585   K0III    L83a
HD139590  15 39 01   -0 18.7   7.500   0.543   0.312   0.297   G0V      L83a
HD140873  15 46 06   -1 48.3   5.393  -0.032   0.000  -0.009   B8III    T86
HD149845  16 37 21   -0 24.8   7.964   1.303   0.682   0.589   K2       L83a
HD157881  17 25 45   +2 06.7   7.540   1.356   0.854   0.768   K7V      L83a
HD161223  17 44 04   +6 03.7   7.435   0.326   0.22    0.24    A2       L83b
HD161242  17 44 13   +5 15.0   7.806   1.284   0.661   0.637   K2       T86
HD163153  17 54 58   -7 44.0   6.926   0.759   0.41    0.35    G8IV     L83b
HD172651  18 41 27   +0 33.9   7.474   1.449   0.767   0.681   K2       L83a
HD175544  18 55 47   +0 15.9   7.395   0.107   0.074   0.077   B2V      L83a
HD185297  19 38 22   +0 20.7   7.216   0.278   0.154   0.166   A7IV     L83a
HD196426  20 37 18   +0 05.8   6.206  -0.087  -0.039  -0.044   B8IIIp   T86
HD196573  20 38 16   +1 01.0   7.885   1.641   0.931   0.955   K5       L83a
HD199280  20 56 18   -3 33.7   6.566  -0.076  -0.034  -0.043   B8Vn     L83a
HD200340  21 03 00   -0 55.5   6.498  -0.099  -0.037  -0.045   B6V      L83a
HD205584  21 36 14   +6 08.2   7.711   1.264   0.617   0.579   K2       T86
HD209905  22 06 39   +2 26.4   6.496  -0.068  -0.011  -0.036   B9       T86
HD215077  22 42 43   +0 04.3   7.177   0.367   0.224   0.230   F0       T86
HD215093  22 42 49   +0 13.9   6.969   0.311   0.187   0.189   F0       L83a
HD218155  23 05 33  +14 57.6   6.783   0.004  -0.01   -0.01    A0V      L83b
HD223963  23 54 03   -9 17.4   7.200   1.581   0.91    0.95    M0III    L83b

sources:  L83a = Landolt 1983a, L83b = Landolt 1983b, T86 = Taylor 1986.


Notes:
HD   315 - HR 11.
HD  7615 -
HD 18145 - SA 94-32.
HD 18175 - SA 94-293.
HD 18369 - SA 94-319.
HD 30544 -
HD 30545 - HR 1534.
HD 31331 - HR 1574 = SA 96-837.
HD 37334 - NSV 2471, probably slightly variable in V but colors stable.
HD 37981 -
HD 40210 - SA 97-257.
HD 50167 - ADS 5533: sep. ~2" with large delta-mag.
HD 65079 -
HD 75012 -
HD 85990 - SA 101-24.
HD 86135 - SA 101-333.
HD 94864 - SA 102-1085.
HD 97991 -
HD118330 - SA 105-214.
HD129956 - 108 Vir = HR 5501.
HD139137 - 14 Ser = HR 5799 = SA 107-298. composite spectrum.
HD139308 - SA 107-35.
HD139590 - SA 107-595.
HD140873 - 25 Ser = HR 5863.
HD149845 - SA 108-827.
HD157881 - K dwarf with large proper motion.
HD161223 - in region of open cluster IC 4665.
HD161242 - in region of open cluster IC 4665.
HD163153 -
HD172651 - SA 110-471.
HD175544 -
HD185297 - SA 111-1496 = ADS 12708: sep. ~1".
HD196426 - HR 7878.
HD196573 -
HD199280 - HR 8014.
HD200340 - HR 8054.
HD205584 -
HD209905 -
HD215077 - SA 114-69.
HD215093 - SA 114-172.
HD218155 -
HD223963 -


References:

Bessell, M. S.  1990, PASP 102, 1181.
Cousins, A. J., and Menzies, J. W.  1993, in "Precision Photometry", Kilkenny
     et al., eds., page 240.
Crawford, D. L., Golson, J. C., and Landolt, A. U.  1971, PASP 83, 652.
Iriarte, B., et al.  1965, Sky & Telescope, 30, 21 (July 1965).
Johnson, H. L., et al.  1966, Comm. Lunar & Planetary Lab., vol. 4, part 3.
Landolt, A. U.  1983a, AJ 88, 439.
Landolt, A. U.  1983b, AJ 88, 853.
Lee, S.  1994, CCD Astronomy, no. 1, page 4.
Menzies, J. W., et al.  1991, MNRAS 248, 652.
Moffatt, T. J., and Barnes, T. G., III, 1979, PASP 84, 627.
Taylor, B. J.  1986, ApJ Suppl. 60, 577.

VSNET Home Page

Return to Daisaku Nogami

vsnet-adm@kusastro.kyoto-u.ac.jp