The DU Experience

Introduction

Variable stars are different from other types of stars in that they experience a phenonmena causing a change of flux (AAVSO, 1997). This means they change in magnitude when observed over a period of time.

There are four classes of variables: pulsating, eruptive, eclipsing, and rotating. Pulsating and eruptive variables vary due to physical properties of the star, while eclipsing bianary and rotating variables vary due to interactions with other bodies. Pulsating variables vary due to regular or semi-regular pulsations of the outer shells of material of the star. Eruptive variables vary due to destructive proccesses of the star, usually one that tears apart the star. Eclipsing binary variables vary due to one star of the two star system eclipsing the other. Finally, rotating variables vary due to chemical or thermal changes in the upper atmosphere of a star.

Mira variables are members of the pulsating class (Querci, 1986). Mira was discovered in 1596 by shepherd David Fabricas. Then in 1603, astronomer Bayer gave Mira the name Omicron Ceti (Fig.1) because it was 15th brightest star in the constellation. Havel observed Mira in 1683 for 50 consecutive days and determined the its period, 331 days (or about period of a year). This set the precedence for the Mira to be a LPV (or a Long Period Variable). In the same year, Mira gained its name “Mira Ceti”, or “magic star of the Baleen constellation” because it was the only known Mira in the sky. In 1704, G. Kirch observed a second Mira-type variable, X Cyg. Soon other observations of Mira- type stars followed.

Fig. 1 Van-Nattan observatory observations of Omicron Ceti

Miras are also M-type stars. This means they are red giants on the Asymptotic Giant Branch (AGB). They are “cool” stars at less than 3000 K for their effective surface temperature. M stars are interesting because they produce some of the heavier elements beyond the iron peak. M stars are also laboratories for stellar atmospheres. M stars orginate when main sequence stars use up their hydrogen fuel supply. The star's outer shells expand at this point and become red giants. At this point the star can become a Mira. In a Mira, the expansion of the star can become unstable due to the radiative energy being less than the gravitational potential energy. The energies do not balance out therefore, the shell material falls back to the star, but at certain point the radiative energy becomes more than the gravitational energy, the star expands again. This process continues until equilibrium is achieved. An average Mira is one solar mass.

During this pulsation, mass is lost (Goldberg, 1986). The first cause of mass loss is particles that are lost to space due to the pulsation. Also the star is going through the transfer to the helium burning process, so the star is going through the s-process. This creates heavier elements due to interactions with the outer shell material. The mass slowly leaks to space. This mass loss is 10-4Msolar/yr at peak mass loss. In the case of Mira variables, oxygen is the main element produced (C/0 = .6). Also with the new elements and the surrounding material, molecules are formed. The main molecules are CO, OH, SiO, water, and TiO. Titanium Oxide is by far the most abundant due to an abundance of oxygen and the s-process creating Titanium.

Due to this mass loss, Mira variables are the target of many Infrared astronomers. Dust particles are best viewed in the infrared because they absorb the heat of the star the particles surround. Infrared radiation is felt as heat by humans, so the particles are observed in the IR part of the electromagnetic spectrum. IR astronomers use the particles to model the system around the Mira. The standard model is a type of planetary nebulae surrounding the dying star. Michelle Creech-Eakman has studied the mass-loss of Mira-type variables and tried to model the proccesses surrounding the variables for her doctoral thesis.

The work shown in this chapter support the work in Michelle's thesis by creating baseline light-curves in the optical. The reason for this is to see what is happening in this geographical area of Denver to the light-curves of Miras. These light-curves were used as comparison light-curves.

Observations

We observed R Aur for three months in early to mid 1996. The observations were part of a independent study under Dr. Stencel by Matti Jalakas, Thomas Hanan, and myself. The observations were made at the Van-Nattan Student Observatory in Chamberlin Observatory located at Observatory Park, Denver. The observatory is a dual telescope facility with a 8" Celestron Cassigrain and 6" Celestron Newtonian finder scope. The camera is a S-Big 5 CCD camera with thermocoolers. The system is controlled by The Skypro on camera and The Sky for telescope control. Finally, there is a filter wheel with two of the standard Johnson filter system filters, B and V. The observations obtained used the B and clear filters at a constant time of 5 secs each. The observations on 2/26/96, 3/20/96, and 4/13/96, or on a three month period. Data for 3/20/96 is presented in Table 1 as an example. Concurrently we observed two constant magnitude comparison stars called SAO 25105 and SAO 25116 on each date. 

Table 1 
Data for 3/20/96						

Star Name		R Aur		Filter/Date		V-3/20/97

Type		Mira Var		Exp		5 secs

Net counts		41593.6		ct/s		8318.72

Comparison Star #1			SAO 25105			

Net Counts		10594.1		ct/s		2118.82

Comparison Star #2 			SAO 25116			

Net counts		64746.5		ct/s		12949.3


Delta M#1		1.484906

Delta M#2		-0.48047

Ave Delta M		0.502216

Data and Analysis

Already dark subtracted, we converted the files from the .st5 format and and data reduction on the platform of Mira. After converting, we did apeture photometry on R Aur and SAO 25105 and SAO 25116 on each date, except for one that did not have an observation of the SAO 25105. We found the net counts and subtracted the background counts to obtain a corrected count. Then using the difference (delta) magnitude equation found in the Introduction, we found the average delta magnitude between R Aur and its comparison stars (Table 2) Once we found the delta magnitude we created a light-curve as in Fig. 2. 

Table 2	
Delta Magnitudes over time for R Aur	

Julian Date	Delta Magnitude
2450140		8.0000
2450163	       -0.4804
2450187		0.2150

Fig. 2 Light-curve for R Aur betwen 2/96-5/96

The light-curve intially does not look like a light-curve consistent with a long period variable, but it is my suggestion this Mira is a rapidly varying maxima type Mira (Querci,1986). It seems to vary quite quickly around its maxima, then drift into its minima. This suggests a period of about 225 days, which is substantially less than a year. Also, the light curve matches the actual observations of the R Aur (Fig.3). So this light-curve could be true.

Conclusions

The light-curve of R Aur seems consistent with a type of Mira that has a period of 225 days. Whether or not this is true must be explored. These observations demonstrate the need for light-curves which are a function of photometry. Right now we are working on correcting this and other light curves by observing the effect of the sky and telescope on images taken in Van-Nattan. Two studies underway to do this are the Atmospheric Extinction study being performed by Matti Jalakas and the Incipient Voltage study being performed by Thomas Hanan. Both studies, if effective will bring a better understanding on what outside effects have on light in Van-Nattan, thus producing more accurate light-curves. Hopefully, these light-curves will help model the system surrounding a Mira.

References