THE STANDARD STAR NEWSLETTER

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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 WorkingGroupChair, Chris Corbally. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .p. 2
Contribution: A Spectral-type CodingSystem: 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 methodofcodingspectral types will become widespread.Finally, Susan Stewart at the Naval Observatory asks our advice about which set of spectrophotometric standards to include in the next volume of the AstronomicalAlmanac. If you have experience in the use of spectrophotometric standards, please be certain to contact Susan with your advice.

Richard Gray, editor grayro@appstate.edu

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 Observatorytools withoutone needingto know about them andthe 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 theyexist in the archive. We now describe the plans that MAST has to make this possible. We hope that this good idea will spreadto 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 “spectralclass”as a cell in three dimensionalspace 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 upto two spectral pecularities to be specified.The overall plan is for MASTto bringover 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/spclass/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 dragandclickmouse action. The user can then downloada table and/or select andorganize download 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

Table 2: Table of Luminosity Classes (LL)

Class	Code
null	00
0	10
Ia	11
Iab/I	12
Ib	13
Ib-II	14
II	15

Class	Code
II-III	16
IIIa	17
IIIab/III	18
IIIb	19
III-IV	20
IV	21
IV-V	22

Class	Code
Va/V	23
Vb/Vz	24
VI	25 (sd)
VII	26 (sd & WD)
VIII	27 (sd & WD)
IX	28 (WD)

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.

DiscussionForum

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.Untilthese surveys observe the entire sky, however, it would be a major step forwardto have an agreed-up calibration region that all surveys should endeavor to do.

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. URLs about WISE:

http://www.astro.ucla.edu/^∼wright/NGSS/http://wise.ssl.berkeley.edu/documents/2007-FactSheet.pdf

A Query

We have received a query from Susan Stewart (sgs@aa.usno.navy.mil) at the US Naval Observatory asking for advice about adding a list of spectrophotometric standards to the Astronomical Almanac. Her emailto Chris Corbally is reproduced below in full. Please respond directlyto 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?

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 anyofthese? If you can offer your suggestions as to the best to include, I’dappreciate your guidance. Thank you!

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/htmlmanuals/technotes/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/. Under “StarsandStellarSystems/LandoltUBVRIStandards/SearchableDatabaseofLandoltStandards/”, we have a utility to search our database of Landolt Standards by RA, DEC, and magnitude withoutput to a PDF file.The coordinates are mean positions for a selected epoch.The direct link is: http://asa.usno.navy.mil/SecH/LandoltSearch.htmlor http://asa.hmnao.com/SecH/LandoltSearch.html.

Contributions to the next Newsletter, due to be “crystallized” in March 2009, will be welcomed at anytime 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.

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