1 

ROTSE: THE SEARCH FOR SHORT PERIOD VARIABLE STARS

COURTNEY FAGG

SOUTHERN METHODIST UNIVERSITY

Dallas, TX 75275

Abstract

We present a search for stars that demonstrate rapidly changing visible light curves.

The data we have used for this search has been procured from the ROTSE-I and III

telescopes’ archival data of multiple nights in April 2000. These thirteen fields

cover eight degrees of the night sky, and are observed for at least two continuous

hours in one night. By using general statistical properties of the light curves, we

were able to narrow our search to obtain clearly-varying light curves. The analysis

has shown to be most effective for light curve variations with periods less than 0.3

days that possess magnitudes greater than 0.1 mag., and mean magnitude values

varying from 10 mag. to 15 mag. Our search has produced 20 variable star

candidates, whose classification will be based solely on their light curve

characteristics. We have observed phenomena such as YY δ Scuti and ZZ W UMa

stars in our data collection. The presence of these short period variable stars prove

the effectiveness of our method of searching.

I. Introduction

Stars with varying magnitudes (known as variable stars) have interested astronomers for

hundreds of years, beginning with the first supernovae with pulsating light variations that

were observed and documented. Since then, thousands of variable stars have been

  2 

discovered and classified based on their peculiarities in variation. The observation of

these interstellar phenomena have allowed astronomers, both amateur and professional, to

learn more about the night sky, as well as the night sky not visible to the unaided eye.

Variable stars are grouped into two major categories: intrinsic, whose stars'

luminosity varies by pulsating and/or shrinking in size due to physical characteristics, and

extrinsic, whose stars appear to vary in magnitude due to an eclipsing companion. Long

period variable stars have been studied for many years because of their apparent

brightness changes that can be easily observed using a telescope, depending on the

magnitudes of variation. The period of these stars can last from weeks up to several

years.

Most recently discovered have been the short period variable stars, whose periods

can last from less than an hour to a few days. These types of variables are more

commonly observed today because of their high-energy outputs and unusual light curves.

Two examples of these types of variables are the RR Lyraes (period: 0.2 day ≤ T ≤ 1.0

day) and δ Scutis (period: T > 0.3 day), which are radial pulsators. Eclipsing binary

systems containing stars relatively close or touching each other also exhibit short period

behavior, and their light curves are generally characterized by a span of higher magnitude

with a small dip of lower magnitude. Binary systems of the W UMa type contain stars

that are presumably so close in proximity that the surfaces are in contact with one another

[1]. The second law of thermodynamics states that the entropy of an isolated system,

which is not in equilibrium, will increase until equilibrium is attained. By nature, heat

  3 

transfers from the body of higher temperature to the body of lower temperature, therefore

the heat, and thus, luminosity, is transferred from the more massive star to the less

massive one until equal temperatures are acquired. According to this law, it would be

expected to see a light curve that is similar to binary systems containing separated stars,

though with time, the dips in magnitude would become similar in size, corresponding to

the equalization of the stars’ temperature and luminosity. Binary systems with stars

whose surfaces do not touch however, such as Algol systems, do not experience this

transfer of heat and mass, and therefore should exhibit light curves that show consistent

dips in magnitude.

Many telescopes, including the Robotic Optical Transient Search Experiment

telescopes (ROTSE-I and III) observe these short period variables. The ROTSE

telescopes have provided the data used in this report.

II. Detector and Data

The ROTSE telescopes generate images taken when photons emanating from a light

source are focused to an image on an array of cells and sends an electric signal based on

the intensity of the source. These signals are usually only a pixel wide, but, for bright

stars, can be spilled over to multiple cells, creating an undesirably bright and distorted

image. Many different signals can be obtained over the span of one night, and with the

use of the operating system Linux and Interface Description Language (IDL), these

collected data inputs can be placed together based on magnitude of light intensity versus

time, creating a light curve. These small but powerful telescopes are distributed around

  4 

the world for international use.

Today, two phases of ROTSE telescopes have been utilized: ROTSE-I and III, with

II as an abortive intermediary step between the two. The telescopes were originally

created to study the optical light emitted by gamma ray bursts (GRB) in deep space, but

now they are used to study optical light from numerous types of sources, including

variable stars.

III. Selection

The data stored in the University of Michigan’s IDL library can be extracted through a

special directory search called “find_burst.” Data is classified according to quantifying

search “cuts” inputted by the user based on Δ magnitude (deltamag), maximum σ

(maxsig), and χ2 (chisquared).

Δ m = mmax - mmin (1)

σmax = (mmax - mmin) / (εmax2 - εmin

2) ½ (2)

χ2 = Σ (Δ m / σmax) ½ (3)

  5 

These cuts indicate the level of error present in each light curve, and some levels

of error proved to be more useful than others. The initial cuts used in the search were:

Deltamag Maxsig Chisquared

0.5 2.0 3.0

0.1 0.0 3.0

0.1 0.0 5.0

0.5 0.0 3.0

0.1 5.0 1.0

1.0 2.0 5.0

1.0 5.0 0.0

0.1 3.0 1.0

TABLE 1 – Cuts in find_burst used in initial image search.

The images shown in Figures 1-5, along with the data displayed in Table 2 display

the light curve data from the best-yielding cuts, including deltamag 0.1, maxsig 5.0,

chisquared 1.0; deltamag 0.1, maxsig 0.0, chisquared 3.0; and deltamag 0.1, maxsig 0.0,

chisquared 0.5. The remaining cuts yielded a couple of these desirable images, however

the majority of the images exhibited light curve characteristics that have high levels of

error, are incomplete, and/or unclear.

  6 

Cuts using deltamag 0.1 yielded the most results: i.e. Δ m = 0.1, σmax = 5.0, χ2 = 1.0

yielded 121 pages of light curves, both desirable and undesirable. The last cut, Δ m =

0.1, σmax = 5.0, χ2 = 1.0, yielded 1436 pages of light curves. Any data showing

incomplete or indistinguishable curves were disregarded. Δ m = 0.5, σmax = 0.0, χ2 = 3.0,

Δ m = 0.5, σmax = 2.0, χ2 = 3.0, Δ m = 1.0, σmax = 2.0, χ2 = 5.0, and Δ m = 1.0, σmax = 5.0,

χ2 = 0.0 yielded the least satisfactory results, ranging from 3 – 37 pages of undesirable

light curves. It had become clear that cuts using deltamag 0.1 would yield the largest

number of light curves, with a maxsig of 3.0 showing the largest number of desirable

light curves.

IV. Catalog of Objects

The twenty objects that were chosen based on their clarity and advantageous

light curves were compared to existing objects catalogued in the SIMBAD astronautical

database according to their right ascension and declination. The matches in the catalogue

had proven to be either a pulsating variable, eclipsing binary, radio source, quasar, or

unidentified. Table 2, shown on the next page, lists the variables with clear light curves

that have close matches with objects already catalogued in SIMBAD.

  7 

Object #† Object Name‡

Right

Ascension (hr. min.

sec.)

Declination (deg. min.

sec.)

Period (Day)

Magnitude Range

657

V* HH UMa

11 4 48.11 +35 36 26.6 0.2 11.06 – 10.85

950

2MASS

J11372169+4255441

11 37 21.75 +42 55 44.6 0.205 11.67 – 11.37

1127

TYC 3012-1895-1

11 13 45.07 +42 39 51.7 0.32 11.84 – 11.67

1199

V* MT UMa

11 33 34.68 +42 58 29.2 0.4 11.95 – 11.67

1212

V* MU UMa

11 35 36.72 +38 45 57.5 0.5 12.24 – 11.77

1222

V* MQ UMa

11 21 41.02 +43 36 53 0.285 11.84 – 11.56

1266

FIRST

J111722.9+394253

11 17 19.72 +39 43 3 0.45 12.13 – 11.82

1357

V* BS UMa

11 25 41.63 +2 34 48.8 0.175 12.28 – 11.93

1459

V* MO UMa

11 13 5.98 +40 21 0.3 0.31 12.07 – 11.71

1521

V* MP UMa

11 20 37.62 +39 21 0.3 0.07 12.205 –

12.09

1885

2MASS

J1116506+3550272

11 16 15.06 +35 50 27.2 0.2 12.85 –

12.46

TABLE 2 – List of identified variables in the collected 2000 April stare data. The nearest

matches in SIMBAD are given.

  8 

Table 3 shows the list of variables with light curves that have either no close or

direct matches with objects already catalogued in SIMBAD.

Object

#†

Object Name‡

Right

Ascension (hr. min.

sec.)

Declination (deg. min.

sec.)

Period (Day)

Magnitude Range

2316

FIRST J113922.2+403640

11 39 28.27 +40 36 32.8 0.255 13.1 – 12.6

2354 _ 11 13 40.03 +42 44 13.8 0.21 12.81 – 12.55

2670 _ 11 14 15.57 +37 18 25.6 0.05 13.07 – 12.97

2827

SDSS

J111055.84+381055.1

11 11 5.45

+38 11 23.5 0.22

13.32 – 13.11

3046 _ 11 17 16.02 +38 57 16.9 0.41 13.37 – 13.1

3102

FIRST

J112148.9+405909

11 21 48.08 +40 59 38.4 0.21 13.38 – 13.12

3121

GB6 B1117+4411

11 20 9.02 +43 53 49 0.31 11.6 – 11.43

3786

FIRST

J111740.0+410628

11 17 34.08 +41 6 49 0.31 13.7 – 13.2

4571 _ 11 3 40.78 +40 26 17.1 0.15 14.1 – 13.5

TABLE 3 – List of variables that are not directly identified in SIMBAD. Those with

object names are of catalogued objects that are located closest to the variables’

coordinates.

  9 

† According to ROTSE-I 2000 April stare data. ‡  According to SIMBAD astronomical database. 

V. Results

Pulsating Variables

FIG. 1 - Seven single-night light curves for pulsating variables from the 2000 April stare

data. Each of these has previously been identified as variable through the SIMBAD

astronautical database. Errors are statistical + systematic.

Pulsating Variables

FIG. 1 - Eight single-night light curves for pulsating variables from the 2000 April stare data.

Each of these has previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Eclipsing Binaries

Pulsating Variables

FIG. 1 - Eight single-night light curves for pulsating variables from the 2000 April stare data.

Each of these has previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Eclipsing Binaries

Pulsating Variables

FIG. 1 - Eight single-night light curves for pulsating variables from the 2000 April stare data.

Each of these has previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Eclipsing Binaries

Pulsating Variables

FIG. 1 - Eight single-night light curves for pulsating variables from the 2000 April stare data.

Each of these has previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Eclipsing Binaries

  10 

Eclipsing Binaries

FIG. 2 - Three single-night light curves for candidate pulsating variables from the 2000

April stare data. Each of these have previously been identified as variable through the

SIMBAD astronautical database. Errors are statistical + systematic.

Radio Sources

FIG. 3 - Four single-night light curves for candidate radio sources from the 2000 April

Pulsating Variables

FIG. 1 - Eight single-night light curves for pulsating variables from the 2000 April stare data.

Each of these has previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Eclipsing Binaries

FIG. 2 ! Three single-night light curves for eclipsing binaries from the 2000 April stare data.

Each of these have previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Radio Sources

FIG. 3 - Four single-night light curves for candidate radio sources from the 2000 April stare data.

Two of these, objects 1266 and 3121, have previously been identified as radio sources through

the SIMBAD astronautical database. Objects 2316 and 3786 have close matches to radio

sources in the database. Errors are statistical + systematic.

Quasar

FIG. 2 ! Three single-night light curves for eclipsing binaries from the 2000 April stare data.

Each of these have previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Radio Sources

FIG. 3 - Four single-night light curves for candidate radio sources from the 2000 April stare data.

Two of these, objects 1266 and 3121, have previously been identified as radio sources through

the SIMBAD astronautical database. Objects 2316 and 3786 have close matches to radio

sources in the database. Errors are statistical + systematic.

Quasar

  11 

stare data. Two of these, objects 1266 and 3121, have previously been identified as radio

sources through the SIMBAD astronomical database. Objects 2316 and 3786 have close

matches to radio sources in the database. Errors are statistical + systematic.

Quasar

FIG. 4 - Single-night light curve for a candidate quasar from the 2000 April stare data.

This has previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Yet to be Identified

FIG. 5 - Four single-night light curves for candidate pulsating variables from the 2000

FIG. 2 ! Three single-night light curves for eclipsing binaries from the 2000 April stare data.

Each of these have previously been identified as variable through the SIMBAD astronautical

database. Errors are statistical + systematic.

Radio Sources

FIG. 3 - Four single-night light curves for candidate radio sources from the 2000 April stare data.

Two of these, objects 1266 and 3121, have previously been identified as radio sources through

the SIMBAD astronautical database. Objects 2316 and 3786 have close matches to radio

sources in the database. Errors are statistical + systematic.

Quasar

FIG. 4 - Single-night light curve for a quasar from the 2000 April stare data. This has

previously been identified as variable through the SIMBAD astronautical database. Errors are

statistical + systematic.

Yet to be Identified

FIG. 5 - Four single-night light curves for candidate pulsating variables from the 2000 April stare

data. None of these have been previously identified as variable. Errors are statistical +

systematic.

Object # ! Object Name " Right

Ascension (hr. min.

sec.)

Declination (deg. min.

sec.)

Period (day)

Magnitude Range

657 V* HH UMa 11 4 48.11 +35 36 26.6 0.2 11.06 #

10.85

950 2MASS

J11372169+4255441

11 37 21.75 +42 55 44.6 0.205 11.67 #

11.37

1127 TYC 3012-1895-1 11 13 45.07 +42 39 51.7 0.32 11.84 #

11.67

1199 V* MT UMa 11 33 34.68 +42 58 29.2 0.4 11.95 #

11.67

1212 V* MU UMa 11 35 36.72 +38 45 57.5 0.5 12.24 #

11.77

1222 V* MQ UMa 11 21 41.02 +43 36 53 0.285 11.84 #

11.56

1266 FIRST

J111722.9+394253

11 17 19.72 +39 43 3 0.45 12.13 #

11.82

  12 

April stare data. None of these have been previously identified as variable. Errors are

statistical + systematic.

V. Conclusion

VI. References

[1] Sterken, C., & Jaschek, C. Light Curves of Variable Stars: A Pictorial Atlas.

Cambridge University Press, 1996.

[2] Levy, David H. Observing Variable Stars: A Guide for the Beginner. Cambridge

University Press, 1989.