# STANDARD STAR - THE STANDARD STAR NEWSLETTER by yurtgc548

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An electronic publication of the Working Group on Standard Stars
an IAU Interdivisional (IV, V and IX) Working Group
No. 44                                                         editor: Richard O. Gray
April 2008                                                         grayro@appstate.edu

CONTENTS:
Editorial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   p.   1
Note from the Working Group Chair, Chris Corbally. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                  p.   2
Contribution: A Spectral-type Coding System: Myron Smith et al. . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                   p.   2
Discussion Forum: A Standard Field for Surveys?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                              p.   4
A Query: Spectrophotometric Standards for the Astronomical Almanac . . . . . . . . . . . . . . . . . . . . . . . .                                                                      p.   5
Online Standard Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   p.   6

From the editor

Longtime readers of this newsletter will note a gradual evolution in the nature of the content, away
from abstracts (although those are still welcome!) towards discussion forums and longer contributions.
This issue features the latest contributions in the discussion on the question of a “Standard Field”
for surveys. See the previous issue of this newsletter for a more detailed description of what is meant
by a “standard ﬁeld”. If you would like to respond or make your own contribution, please send
your response directly to me at grayro@appstate.edu, and I will immediately publish it on the
Standard Star Newsletter webpage http://stellar.phys.appstate.edu/ssn. Any responses will
also be printed in the next newsletter. In addition to that, we have a contribution from Myron Smith
on a spectral-type coding system which will be incorporated into MAST’s (Multi-Mission Archive at
Space Telescope) search engine so that the user will be able to enter spectral-type ranges, including
most of the common peculiarities, and get back a list of observations held at that data center. The
hope is that this new, versatile method of coding spectral types will become widespread. Finally, Susan
to include in the next volume of the Astronomical Almanac. If you have experience in the use of
Richard Gray, editor
grayro@appstate.edu

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A Note From the Chair
International Astronomical Union
Working Group on Standard Stars (WGSS)

Recently I was reminded of the discussion held by our WG-SS some 18 months ago during the Prague
IAU General Assembly. The reminder came through the General Secretary asking me to proofread
the brief report of our meeting. In the discussion, it “was felt that, with the development of new
photometric and spectrophotometric systems by the large surveys, there was even more need for
communication among the various survey teams and from these teams to the individual researchers,
who would use the survey databases.”
There were some practical proposals made at the time to further such communication. The idea of
having a ”standard ﬁeld for surveys” emerged explicitly only later, but it does seem one with the most
potential. So I do encourage you to read what has been exchanged so far and to continue to voice your
opinion and practical suggestions about a standard ﬁeld through comments sent to our SSN editor.
Chris Corbally
corbally@as.arizona.edu

Contribution

A Spectral-type Coding System for Targeted Retrievals of Stellar
Spectral Classes
Myron Smith1 , Richard Gray2 , Christopher Corbally3 , Randall Thompson1 , and Inga
Kamp4
1
Multi-Mission Archive at Space Telescope
2
Appalachian State University
3
Vatican Observatory Research Group
4
University of Groningen

Have you ever initiated what turns out to be the laborious project of trying to ﬁnd out what stars
of a given spectral type are present in a NASA or any other archival database? It need not be a
time-consuming chore. Yet up to now it has been so because databases are generally not designed
with simple object property search criteria in mind to satisfy researchers that want to search on basic
spectral criteria. The International Ultraviolet Explorer once took a small step in this direction by
asking their Guest Observers to select among an “Object class” for their observing targets. However,
these were subject to the ambiguity of whether, for a B2e star the GO selected the “B0-B2” or the
“Be” class for the object. Since then, only one other spectroscopic mission has copied the IUE’s
crude system. Others have backed oﬀ entirely, leaving the archival researcher the choice of manually
checking the spectral types and slowly accumulating bins to his/her liking. But it need not be this
way! We describe in brief a spectral class system in which you the user can use the web and Virtual
Observatory tools without one needing to know about them and the system we describe, to ﬁnd lists of
objects that satisfy your speciﬁcation of a range of spectral types, luminosity classes, and peculiarities.
The concept is for the user to go to the website of a large astronomical archive data center, like the
Multi-mission Archive at Space Telescope (MAST) and ask for lists of stars grouped by their spectral
classiﬁcation. The results page will allow one to drill deeper and get the mission datasets as they exist
in the archive. We now describe the plans that MAST has to make this possible. We hope that this

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good idea will spread to other data centers and databases, thereby enabling queries for stars for many
instruments, space-borne or ground-based in the future.
First, we deﬁne a “spectral class” as a cell in three dimensional space that describes a star’s spectrum
in terms of its spectral type, luminosity class, and well deﬁned peculiarities. A star’s spectrum may
be uniquely and unambiguously assigned to a spectral class cell by means of an ascii string known as
its spectral classiﬁcation. We also deﬁne “spectral class system” as a series of 0-9 digital codes that
expresses this classiﬁcation. Our experience shows that we need at least two digits to describe stars’
spectral types, subtypes, luminosity classes and peculiar codes, so our coding system will take the
form TT.tt.LL.PPPP. Here we ﬁnd we have to have four P codes (P1, P2, P3, P4) just to enable one
to allow up to two spectral pecularities to be speciﬁed. The overall plan is for MAST to bring over the
“best” spectral types (currently we have a choice of two possible sources of spectral sources, and we
are in the process of comparing them). These “spectral string types” are then mapped to our system
codes and stored in a database. A user comes along and makes a request at a MAST web portal,
perhaps with a future url like http://archive.stsci.edu/sp class/search.php . This initiates a
search to the database and the codes are used to cull a list of stars that satisﬁes the spectral class
range requested. The web portal may be a familiar search form, looking somewhat like an airline
reservation page or a contour plot of type versus luminosity classes that allows one to select ranges by
datasets by object(s).
Table 1 shows spectral type codes of the form TT for all practically recognized spectral types on the
HR-Diagram, including the recent cool “LTY” types, types that are still hypothetical (NS), and some
additional room for expansion; subsubtypes like B9.5 are not recognized. As one moves to faint stars
which are poorly recognized, we will have many stars with no known subtypes, and to keep the system
simple we give these “null” types a coding of 00. For example, a spectral type of only “A5” will have
LL = 00, and PPPP = 0000. Table 2 describes the LL classes.

Table 1: Table of Spectral Types (TT)
Sp.   Code   Desc.    Sp.   Code        Desc.       Sp.   Code     Desc.       Sp.   Code      Desc.
–     00    Null      Y     19      Future cool    SN     29    Supernovae    DZ     45      DZ WDs
Brown Dwarfs
O      10    O-type    ?     20     Reserved for    sd     30     subdwarf     DQ     46      DQ WDs
Future BD’s                    w/o type
B      11    B-type   CR     21         R-type      sdO    31       O-type     PG     47      PG 1159
Carbon stars                  subdwarfs                     stars
A      12    A-type   CN     22         N-type      sdB    32       B-type     D?     48     Reserved for
Carbon stars                  subdwarfs                 new WD type
F      13    F-type   CJ     23         J-type      sdA    33       A-type     D??    49     Reserved for
Carbon stars                  subdwarfs                 new WD type
G      14    G-type   CH     24        CH-type      D      40    white dwarf   NS     50      Neutron
Carbon stars                   w/o type                     stars
K      15    K-type   CHd    25    Hydrogen deﬁ-    DA     41     DA white     WR     51     Wolf Rayet
cient Carbon                    dwarfs                    w/o type
stars
M      16    M-type    S     26      S-type stars   DB     42     DB WDs       WN     52     N-seq WR
stars
L      17    L-type   NV     27        Novae        DC     43     DC WDs       WC     53     C-seq WR
stars
T      18    T-type   ??     28      Open for       DO     44     DO WDs       WO     54     O-seq WR
future                                                   stars

Of course the great challenge to any spectral classiﬁcation scheme is to avoid needless complexity from
the many possible peculiarities. To avoid one extreme, we need to ignore the reductionism behind
the statement that “all stars are peculiar” – otherwise our data users will often get back retrievals

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Table 2: Table of Luminosity Classes (LL)
Class   Code   Class       Code   Class   Code
null    00     II-III      16     Va/V    23
0       10     IIIa        17     Vb/Vz   24
Ia      11     IIIab/III   18     VI      25 (sd)
Iab/I   12     IIIb        19     VII     26 (sd & WD)
Ib      13     III-IV      20     VIII    27 (sd & WD)
Ib-II   14     IV          21     IX      28 (WD)
II      15     IV-V        22

of 0 or 1 object, and that won’t do. On the other hand, we know that some users are going to want
to specify Bpe stars, and further, they won’t want their results confused with requests for Bep stars.
This will require several P codes. To keep this managable, we can hide an implicit ”P0” code that
is determined by the spectral type and sets the P3P4 codes. So we can make P3P4 codes that apply
only to O and B stars, a second group of P3P4 codes for A and F stars, etc. We further simplify the
P codes by making P1 and P2 codes the same and universal for all spectral types. For example, we
can specify P1 =1 to denote composite spectra and P2 = 3 for emission lines; spectra of stars across
the HD Diagram are subject to these anomalies. Space forbids printing out the PPPP tables we have
constructed, but interested parties may contact the authors to obtain a copy of the IVOA Design
Note that completely speciﬁes the system. However, the goal is to keep these details out of the hair
of all but a few “power users” who may want to retrieve highly customized lists of stars with common
combinations of peculiarities. For the rest of us, it will be click and play.

Discussion Forum

A Standard Field for Surveys?
In issue #43 of this newsletter, we brought to the Standard Star community the idea of a Standard
Field for surveys, and solicited comments. This idea had its genesis in the course of an discussion
among members of the organizing committee of IAU Commission 45 (Stellar classiﬁcation). The
motivation for this concept is the large number of large-scale surveys currently underway or in the
planning stages (SDSS/SEQUE, RAVE, Pan-STARRS, LSST, Gaia, WISE, etc.) Since most of these
surveys are characterizing the stars they observe in some fashion, it occurred to us that it would be
very useful for all surveys, as far as it is possible and compatible with their designs, to observe a
carefully chosen standard ﬁeld. The argument for this idea is more fully developed in SSN43. Here
we reprint in full the responses of a number of astronomers:

Ted von Hippel, Nov 27, 2007:
Research Scientist
Department of Astronomy
University of Texas at Austin
I think this is a good idea. In 20 years, I suspect that much of the calibration will be higher precision
and take less observing time, since we will calibrate the entire sky. This is now almost being done
by photometric surveys, and hopefully it won’t be long before it can be done by spectrophotometric
surveys. Until these surveys observe the entire sky, however, it would be a major step forward to have
an agreed-up calibration region that all surveys should endeavor to do.

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My simple suggestions are that this ﬁeld be
1. equatorial,
2. have b > 10◦ since many types of surveys avoid the Galactic plane,
3. very low reddening,
4. be modest in size (e.g. 10 sq deg) so as not to increase survey overhead substantially, but still be
large enough to contain a variety of stellar types, and
5. contain an open or globular cluster to benchmark any ﬁlter system against stellar evolution.

Nancy Grace Roman, Dec 26, 2007:
I convinced myself many years ago when classifying MK-type spectra that no two stars are identical.
That does not make standards useless. No two feet are identical but we would have extra diﬃculty
selecting shoes if there were no sizes connected to them. The same is true of stars. There are many
problems in which an approximate description of an object is all that is needed. For determining
whether a star is a member of a cluster, you do not need its absolute magnitude to 0.01 magnitude
but you do need it within 0.3m. For that, a reference to a standard is adequate. Most of us know
what we visualize when we see a paper about L-type stars. If we are looking for hot young stars, we
skip that paper. Thus I feel that standards and standard ﬁelds are desirable. (I would vote for more
than one. The Landolt ﬁelds might be a good choice.) Low latitude ﬁelds should be included so that
we have information on young stars.

Robert McMillan, March 31, 2008
Lunar and Planetary Laboratory
University of Arizona
bob@lpl.arizona.edu
Thanks for running the Standard Star Newsletter. I would like to oﬀer an alternative location for
standard star ﬁelds. Spacecraft that survey the entire sky, such as IRAS did and WISE will, cover
the ecliptic poles frequently. The heavy repetition in these areas at wavelengths that are considered
exotic by ground-based observers make them important for astrophysics. The ecliptic poles also have
long observing seasons when observed from the ground by virtue of their high and low declinations
(±66.5 degrees).
As a member of the Science Team of WISE (Wide-ﬁeld Infrared Survey Explorer) that is scheduled
to be launched in November 2009, I thought I should draw the community’s attention to these areas.
http://www.astro.ucla.edu/∼wright/NGSS/
http://wise.ssl.berkeley.edu/documents/2007-Fact Sheet.pdf

A Query

We have received a query from Susan Stewart (sgs@aa.usno.navy.mil) at the US Naval Observatory
Her email to Chris Corbally is reproduced below in full. Please respond directly to Susan at the email
address above. She hopes to pull this section of the Almanac together by early June. She also seeks
guidance on what information to list about the spectrophotometric standards in the Almanac – i.e.
is it necessary to list anything more than the star name, RA, DEC, Vmag and spectral type?

5
I am writing you in your capacity as the chairman of the standard stars WG. I think I’ve had some
conversations with you before concerning the photometric standards that we list annually in the
Astronomical Almanac. We would like to add a list of spectrophotometric standards. The following
have been suggested. We have limited space so would like to include only one of the following. Does
your WG discuss any of these? If you can oﬀer your suggestions as to the best to include, I’d appreciate
Susan Stewart
US Naval Observatory
1. Those used by ESO at http://www.eso.org/sci/observing/tools/standards/spectra/.
2. The list given on a web site called Standard Objects for Astronomy at http://sofa.astro.
utoledo.edu/SOFA/domains.html.
3. An amalgamation of two old RGO lists at http://www.ing.iac.es/Astronomy/observing/
manuals/html manuals/tech notes/tn065-100/workflux.html.

Online Standard Stars from the Astronomical Almanac
Also from Susan Stewart, US Naval Observatory:
I wanted you to be aware of a relatively new service available on the website version of the
almanac: http://asa.usno.navy.mil/ and the mirror site http://asa.nao.rl.ac.uk/. Un-
der “Stars and Stellar Systems/Landolt UBVRI Standards/Searchable Database of Landolt
Standards/”, we have a utility to search our database of Landolt Standards by RA, DEC, and mag-
nitude with output to a PDF ﬁle. The coordinates are mean positions for a selected epoch. The direct
LandoltSearch.html.

Contributions to the next Newsletter, due to be “crystallized” in March 2009, will be welcomed at
any time by the editor (grayro@appstate.edu). Any qualiﬁed contribution received will be immediately
published on the Standard Star Website, and then appear in the next newsletter. Contributions may
be sent via email to grayro@appstate.edu using the following template.

WHEN SUBMITTING AN ABSTRACT, PLEASE USE THE FOLLOWING TEMPLATE IF POSSIBLE:

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