Abstract

Double Stars are a pair of stars that appear visually congruent when viewed from Earth. They can be classified as either binary or optical doubles. The former is a gravitationally bound system with a consistent pattern of motion, while the latter is a system that only appears close together. In this paper, the star systems of WDS 18006+5841 STI, WDS 20364-0550 HO 279, and WDS 05439+5548 HJ 2274 were analyzed to determine whether these orbital paths aligned with patterns typical of binary doubles or optical doubles.  Telescope data was requested from the Las Cumbres Observatory and further studied in AstroImageJ and Afterglow to derive an updated separation between the two stars and pitch angle, which was then examined in conjunction with previously published historical data. The research in this paper is intended to build upon the information surrounding the movement of binary star systems, and to contribute to currently existing historical data, which will assist future researchers in similar endeavors.

Introduction

Double stars are termed as two stars that appear close together from Earth; they can be classified as either binary doubles or optical doubles. Binary doubles are defined as a system of two or more stellar components with a gravitational bond, while optical doubles are any pair of stars that incidentally appear close to each other, but are too distant to share a gravitational relationship. Optical doubles have been distinguished through means of measured rectilinear motion, divergent proper motion, or parallax estimates of distance from the Sun [1]. The usage of the term binary star to refer to a pair of stars with a gravitational attraction originates from William Herschel’s Catalogue of 500 new Nebulae, nebulous Stars, planetary Nebulae, and Clusters of Stars; with Remarks on the Construction of the Heavens (1802), where Herschel states “It is easy to prove, from the doctrine of gravitation, that two stars may be so connected together as to perform circles, or similar ellipses, round their common centre of gravity.” [2]

The observation of double stars has a variety of use cases. Some observers use double stars as test objects for telescope angular resolution, [3] and the analysis of the motions of double stars may yield the determination of stellar masses and other properties. There are additionally cases of “fragile” binaries, where the stars in the system are far apart, as well as “interacting” binaries, where one star draws matter from the other. Binary systems remain crucial to understanding stellar formation models [4] and their investigation is also one of the few ways to obtain the mass of a star.

The stars studied in this paper are: WDS 18006+5841 STI 2366, WDS 20364-0550 HO 279, and WDS 05439+5548 HJ 2274, located at coordinates: 18 00 33.71 +58 40 56.1, 20 36 24.69 -05 50 19.2, and 05 43 55.41 +55 48 18.9 (measured in arcseconds), respectively. All three of the aforementioned systems contain a visually identifiable primary star, the brighter star of lesser magnitude, and a secondary star, the dimmer star of greater magnitude.

The primary objective of this paper is to record a new separation and pitch angle in the system of interest to add to pre-existing data. This allows future researchers interested in these systems to ascertain the classification of the star system by observing the change in position over time.

Objective

By combining the analysis of the positional measurement derived from new telescope data and previously published historical measurements, the classification of a double star system can be classified as either physical or optical based on predicted orbital behavior. If previous and current research suggest a clear pattern of orbital behavior between the primary and secondary star, the system has high likelihood of being binary.

Instruments Used

The instrument utilized in this study was the Planewave Delta Rho 350, remotely accessed courtesy of the Teide Observatory in Tenerife. The instrument is a 0.4-meter RCS telescope with a two-element optics system mounted on an LCO equatorial C ring mount. The mount provides a tracking accuracy of approximately one inch, it has some drift over long unguided exposures, so exposures are usually limited to intervals of less than two minutes. The instrument’s basic optics include primary and secondary mirrors, a maximum slewing speed of 10 degrees per second, and a blind pointing accuracy of around thirty inches. [5] The telescope images were taken using a Bessel-B filter, with an exposure time of 2 seconds. Ten exposures for each star system were taken to account for averaging values.

Measurements and Methodology

A star’s location refers to its position in the sky, identified using a coordinate system. In astronomy, the coordinates are referred to as right ascension and declination. Right ascension refers to how far east the star is from the vernal equinox. The declination refers to how far north or south the star is from the celestial equator. By recording the right ascension and declination over multiple observations over many years, astronomers can determine any change of position, which is crucial considering this will establish if a double star is physical, i.e. gravitationally bound and moving together, or not.

The systems of interest (18006+5841 STI 2366, 20364-0550 HO 279, 05439+5548 HJ 2274) were selected from Stelle Doppie, an online database containing information on double stars gathered from the Washington Double Star Catalog. The following search parameters were used to filter the pool of unclassified systems for research purposes: primary magnitude between 9-11, a delta magnitude of less than 4, a separation between 5-10, and the right ascension indicating visibility during the month the pictures were taken.

Telescope images of the system were requested from the Las Cumbres Observatory, a network of robotic telescopes, and were displayed in the AstroImageJ desktop application and adjusted for visibility. The AstroImageJ application allows for measurement of the separation and pitch angle between the primary and secondary star.

Figure 1: STI2366 Measurement in AstroImageJ

The position angle and separation measurements of ten images for the same system were taken and averaged to account for precision errors and scintillation. Afterwards, the pitch angle and separation were converted to x and y points to be graphed on a coordinate plane. The formulas used are the following:

Data

The separation and position angle for each image file were measured in AstroImageJ, as recorded in Table 1-3, before being averaged for a final measurement in Table 4. Standard error calculations are additionally included in Table 4 for accuracy.

Table 1: STI 2366 Separation and PA Calculations

Table 2: HO 279 Separation and PA Calculations

Table 3: HJ 2274 Separation and PA Calculations

Table 4: Averaged values across ten images

Results

The newly determined measurements from AstroImageJ are plotted in conjunction with historical measurements, as shown with system STI 2366 in Figure 2. The system’s horizontal axis is representative of right ascension in units of arcseconds, while the system’s vertical axis indicates declination in units of arcseconds. The new measurement is indicated in orange, while the historical measurements are plotted in blue. A visual comparison of the new measurement with previous historical measurements may allow for a pattern of motion to be observed.

Figure 2: Plot of STI 2366 with current and historical measurements

In Figure 2, the current measurement indicates a motion of moving away from the primary star (The primary star is not located on the above graph, but is assumed to be at the origin, [0,0]). Analyzing the pattern of motion from historical data with time as a reference, however, does not support evidence of a specific pattern of motion away from the primary star. The right ascension values in the system fluctuate without fitting a pattern of sustained movement away from the primary star. This suggests that system STI 2366 is not physical and is not gravitationally bound.

Figure 3: Plot of HO 279 with current and historical measurements

In Figure 3, the previous historical measurements are relatively scattered with the right ascension value ranging from 1.213695 arcseconds to -0.57911 arcseconds (most recent measurement). It is likely that the earliest measurements taken of the system were inaccurate due to the limitations of historical astrometric techniques and instrumentation. The first measurement of HO 279 in 1888 recorded a declination of 6.579644; which is vastly different from the succeeding measurements taken afterwards – supporting the notion of historical inaccuracies. From more recent data, the right ascension is shifting leftwards, away from the position of the primary star. From these observations, system HO 279 may either be potentially gravitationally bound or simply drifting apart over time.

Figure 4: Plot of HJ 2274 with current and historical measurements

In Figure 4, there is a concentrated cluster of points around -6 arcseconds on the horizontal axis and -4 on the vertical axis. Over time, the right ascension has steadily increased, moving closer to the position of the primary star. The right ascension of the system has primarily fluctuated between -6.2 and -5.89, but recent measurements have indicated a trending increase in the declination. To note, an outlier at the far right of the plot is visible at around (-1.31, -1.509). This plot point was recorded in 1831, the first recorded observation of HJ 2274 accessible in the Washington Double Star Catalog. It is likely that this outlier is another result of historical inaccuracies from antiquated telescoping technology. The observations of HJ 2274 suggest that the system may be gravitationally bound due to a trend in recent measurements, but a more formal conclusion requires more data for verification.

Figure 5: A model of a simulated orbit for STI 2366

In Figure 5, an instance of measurements is fitted to an ellipse to model an example of orbital behavior for system STI 2366. It is important to note that Figure 5 is intended as a visualization of an orbital pattern, not a precise prediction that accurately evaluates the relationship between the primary and secondary star.

Conclusion

From observing the pattern of motion for each star system, a prediction on the classification of each double star was made. The calculation of the new position of system 18006+5841 STI does not suggest a concrete classification as either a physical binary or optical double but is predicted as not gravitationally bound due to the lack of a sustained pattern of motion. Systems HO 279 and HJ 2274 indicate the potential to be binary but require more data points to be classified as such. It is important to note that the predictions in this paper are not solid confirmations on the classification of the systems and that more measurements are required to make a solid judgement. Currently, a determination for each system’s classification would greatly benefit with the addition of further data.

Acknowledgements

This research was made possible by the Washington Double Star catalog maintained by the U.S. Naval Observatory, the Stelledoppie catalog maintained by Gianluca Sordiglioni, Astrometry.net, and AstroImageJ software, which was written by Karen Collins and John Kielkopf.

This work has also made use of data from the European Space Agency (ESA) mission Gaia (https:// www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work makes use of observations taken by the 0.4m telescopes of Las Cumbres Observatory Global Telescope Network located in Santa Barbara, Australia, South Africa, Texas, Hawaii, and Chile.

We would also like to thank our research mentor, Dr. Anna DeJong, for their guidance and dedication to our project over the course of the spring and fall semesters.

Contact: kaelyn.sun@howardcc.edu, jennifer.arnold@howardcc.edu, mariam.hassen@howardcc.edu