Methods of Determining Relative Age of Young Stellar Objects Zak Schroeder, Nathan Stano, Graham Kozak Teacher: Ardis Herrold

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1 Methods of Determining Relative Age of Young Stellar Objects Zak Schroeder, Nathan Stano, Graham Kozak Teacher: Ardis Herrold ABSTRACT The main goal of the project was to study various types of Young Stellar Objects, such as T Tauri Stars and Herbig Ae/Be stars, to compare the effectiveness of various systems of age categorization. Spectra of the stars were taken at Kitt Peak National Observatory near Tucson, Arizona, using the Coudé Feed Spectrograph. These spectra were analyzed for the presence of anomalies common to young stars, as well as chemistries. Spectral classes were determined by comparisons with stars in the Jacoby Atlas. LRGB images were taken remotely with the New Mexico Skies telescope near Cloudcroft, New Mexico. These images were analyzed for structural changes in the surrounding nebulae, as well as deviations in magnitude. Also, B-V and V-R photometric data were analyzed for use in correlations and plotting stars on an H-R diagram. It was found that determining relative age by plotting the stars on the H-R diagram proved the most effective. Also, a linear correlation was found between the emissive severity of the stars and the absolute magnitude of the stars. INTRODUCTION Young Stellar Objects (YSOs) are variable stars of about ten million years of age and rapidly fluctuate with temperature, spectral type, wind outflows and inflows and magnitude. Some also have spectral anomalies which may indicate the presence of circum-stellar dust. Spectral types of these stars range from B type to K and they have irregular periods, varying by up to four magnitudes. They have associated nebulosity where changes have been observed over the years. On the H-R Diagram, they are on one of the Hayashi Tracks, leading into the main sequence. The goal of this study is that by studying these stars spectra, imaging data, and photometric data, these stars can be placed more precisely into the evolutionary tracks of young stars. A secondary goal is to gain insight into possible mechanisms that explain the peculiar chemistries and changes in YSOs. Four types of YSOs were observed: T Tauri, FU Orionis, Variable Stars of Orionis (VSOs) type, and Herbig Ae/Be stars. T Tauri and FU Orionis stars are three or less solar masses, but FU Orionis are more eruptive. Herbig Ae/Be stars are of four to eight solar masses. VSOs represent the earlier spectral classes such as B and A type. As with all stars at this stage of evolution, nuclear fusion is not sustained, thus outflows, magnitude, temperature, and many other aspects vary. PURPOSE The main purpose of the project was to observe many types of Young Stellar Objects, such as T Tauri stars and Herbig Ae/Be stars, to compare the effectiveness of several methods of calculating relative age. Once an age rank was decided upon, it was compared to various characteristics of YSO s to see whether any distinct correlations The RBSE Journal V1.6

2 existed. If so, these could be used in future studies of YSO s as a means to determine their evolutionary state. OBSERVATIONS AND DATA REDUCTION Spectral data were obtained at the Coudé Feed Spectrograph at the Kitt Peak National Optical Astronomy Observatory. Spectra of the target stars were compared to the spectra of known stars in the Jacoby Atlas in order to determine the spectral type and an estimate of star temperature. Because these are variable stars, published spectral class data were also obtained from the SIMBAD astronomical database for comparison. The spectra were analyzed for certain elements that showed the various theoretical features present in these stars, including the possibility of accretion disks, P Cygni profiles in hydrogen and calcium lines, the Balmer decrement, emission lines and buried lines. Spectra were also examined for lines of the elements iron, oxygen, helium, hydrogen, and calcium. Photometric data were obtained through the New Mexico Skies program. More than fifteen hours of observing time in were used to make images of nine stars over the course of several months. They were taken with 14 telescope and CCD camera to facilitate visual comparison of the nebulae s structure. Images were made in the luminance, R, V and B bands. Finder charts of the target stars were created using the ALADIN online database in order to determine the stars approximate magnitude under red, blue, green, and luminance filters. The blue, green, and red magnitudes were calculated by opening the respective colored filter image with the program Image J. The magnitudes of several comparison stars from the Aladin Sky Atlas and the USNO Catalogue were used to calculate the magnitudes of the observed stars. Once the B, V (green), and R magnitudes were calculated, they were simply subtracted from one another to find the B-V, V-R, and B-R indices. Age Determination Method 1: the H-R Diagram Color Indices By searching various literature references to the stars and the molecular clouds in which they reside, a list of approximate distances to these stars was compiled. This, together with the measured apparent magnitudes, was used to calculate absolute magnitudes via the distance modulus formula. Once absolute magnitude had been calculated, the stars were plotted on an H-R Diagram using their B-V index as the horizontal axis. For comparison purposes, about 3000 stars were also plotted on the diagram using data obtained from the Catalog of Nearby Stars 1. Positions on the H-R Diagram and their relation to the Hayashi Tracks and the Main Sequence were used to determine their relative stage in the evolutionary sequence of stars. Because YSOs tend to be heavily embedded in dust or gas, they are reddened somewhat from their actual magnitudes. To compensate for this, the Balmer decrement was measured for each star. The Balmer decrement was measured by calculating the equivalent width of the line H gamma (4340 Angstroms) was measured for both the YSO spectra and the corresponding Jacoby stars of the same spectral type. They were then divided to get a ratio and find the percent deviation. The deviation was then applied to the 1 Nearby Stars, Preliminary 3rd Version (Gliese+ 1991) The RBSE Journal V1.6

3 expanded (non-logarithmic) value of the magnitudes to adjust the V and B magnitudes accordingly to account for reddening. Using a reference H-R Diagram with marked Hayashi Tracks, the positions of the YSO s were analyzed. The farther the star was along on its Hayashi Tracks, the older it was ranked and a relative age list was made. Age Determination Method 2: Degree of Emissivity Another way to evaluate how evolved a young star may be is to consider the degree to which it shows emission lines of certain elements. Stars with more and stronger emission lines were considered to be generally younger than those with less, because most normal main sequence stars do not show emission lines. For instance, very young stars show an abnormally strong lithium absorption line, whereas main sequence stars do not show lithium. However in this project most of the red end spectra were not obtained since poor weather conditions prevented it. Other lines which were evaluated are iron, oxygen, helium, hydrogen, and calcium. Previous studies 2 suggest that those stars with strongest emission show dominant iron I (4063 and 4132 A) and helium I (5876A) lines. As emission weakens, iron II (4924 and 5018A) lines appear, then calcium II (8500, 8545, and 8665A) and finally oxygen I (6300 and 6363A). The weak line T Tauri stars show only the Balmer lines in emission. Weak line stars may be the most evolved of all. Using all of the above observations the stars were ranked in relative age. Age Determination Method 3: Other Spectral Anomalies Besides specific chemistry, the presence of certain other features in the spectra and in the optical images might indicate relative age. As a star evolves, it will gradually emerge from its nebula of gas and dust. Early on in its development, it may show spectral anomalies that indicate mass inflows and outflows. P Cygni profiles indicate mass outflows, while inverse P Cygni profiles indicate mass inflows, jets are indicated by high speeds (1000 km/s) and winds by lower speeds (100 km/s). Collimated jets or circumstellar disks may emerge. The Balmer decrement was measured for as many of the major hydrogen lines as were visible. In some cases these lines were buried, which are an emission line embedded in an absorption line, indicating an emission nebula in front of the star. In other cases they were seen in emission or absorption. The presence of a bow shock nebula versus a photo-ionized region was determined by observing the slope of the Balmer decrement as well as the presence of iron lines. Photo-ionization can be identified by a flat Balmer decrement and an absence of iron II lines, and bow shocks can be identified by a sloped Balmer decrement and a presence of iron II lines. Therefore, stars with bow shocks should be younger, due to their severe Balmer decrements and presence of emission lines. These were detected in the spectrum by observing P Cygni profiles in calcium and iron lines. To provide an overall picture, summary cards of each star were made with a complete set of observations for each star. Composite RGB images were also constructed for each star using the Photoshop Liberator to visually evaluate their relative age. If no RGB image 2 Jaschek, Carlos, and Mercedes Jaschek. The Classification of Stars, The RBSE Journal V1.6

4 could be made, a luminance image was substituted. Features such as bow shocks could then be visually identified. The younger stars may appear more optically obscured and reddened. Luminance images of some YSO s were made about 1 month apart in order to look for changes in the nebulae around the stars. To control variables, the same exposure time was used for each image. If the star is more variable, it should be younger. We obtained multiple luminance images of seven stars, and looked for structural changes in all seven stars. Upon close inspection, no great difference in structure was noticed of the nebulae, and thus it can be concluded as an imprecise way for determining age. DATA In the spreadsheet General Spectral Data, the Spectral Features column lists the spectral features present in the spectra for each star, and features are abbreviated; PC denotes a P Cygni profile and thus an outflow. The measured wind speed values appear next to the notation, BS indicates a bow shock, DP denotes a double peaked line suggesting a circum-stellar disc, and PI denotes photo-ionization. The columns labeled Emission red/blue indicates the presence of emission lines in the blue or red end of the spectrum. In the spreadsheet Balmer Decrement Data, numbers such as 4100 is a measure of wavelength in angstroms, while the numbers below that such as 4.1e-14 is a measure of flux in lumens. When an asterisk appears in the table, published values were substituted for incomplete data, or in the case of V1321 Ori, observed values were used, but could not be de-reddened due to sub-par spectra. The RBSE Journal V1.6

5 Emission Lines Spectroscopy Spectral Spectral Features Star Type Effective Temp Star Type Blue Red (K) DG TAU G0Ve BS; Jet* T Tau III NA 5920 DR TAU F0V YY Ori III NA 7240 FU ORI G4V PI FU Ori Ca II; O III NA 5730 HBC 243 B0Ve PC= 243, 308km/s HAeBe III NA HBC 730 B4Ve HAeBe Ca II; O III Ca II HD B6Ve DP HAeBe Ca II; Fe II; O III Ca II R MON F5Ve INA Ca II; Fe II, O III NA 6540 RY TAU K1V PI T Tau No NA 5010 T TAU G4Ve BS T Tau III Ca II 5730 V0380 ORI G0Ve DP INAT III NA 5920 V594 CAS B6Ve PC = 185, 144km/s; DP; Jet HAeBe III Ca II V1321 ORI G8V* T Tau Ca II; Fe II; O III NA 5490 V1331 CYG K0Ve PC = 247, 185km/s; BS T Tau III Ca II 5240 XY PER A5V DP, PI HAeBe No Ca II 8620 Photometry Dereddened Values Star R (Red) Mag V (Green) mag B (Blue) Mag B-V V-R B-R Abs. Mag DG TAU NA NA NA NA NA NA 4.77 DR TAU NA NA NA NA NA NA 1.20 FU ORI HBC HBC HD R MON RY TAU T TAU* V0380 ORI V594 CAS V1321 ORI* NA NA NA NA NA NA 0.78 V1331 CYG NA NA NA NA NA NA 2.01 XY PER Balmer Decrement Equivalent Jacoby % Deviation From Stars Width Equivalent Width Jacoby DG TAU DR TAU FU ORI HBC HBC HD The RBSE Journal V1.6

6 R MON RY TAU T TAU V0380 ORI V594 CAS V1321 ORI NA NA V1331 CYG XY PER V594 CAS, a star with a reflective nebula and jets RY Tau, a star with a reddish nebula and a fine example of photo ionization XY Per, a star with a white nebula and a prime INSERT example SPREADSHEET of photo ionization HERE INSERT YSO PICS HERE HBC 730, a star completely embedded in a reflection nebula The RBSE Journal V1.6

7 ANALYSIS AND DISCUSSION Age Determination Method 1: the H-R Diagram Color Indices When corrected for reddening caused by nebulosity, it turns out to the most effective, lining up well with the Hayashi Tracks. However, the weakness inherent in this method lies in its use of the H-R Diagram. To accurately calculate absolute magnitude, a relatively accurate measurement of distance is needed. If a published distance measurement was off by some number of parsecs, it could drastically change the value for absolute magnitude. Also, if the star was obscured to the point where the magnitude measured was too dim, the B-V values could be skewed. Despite these problems, this method seems the most effective in rating stars according to relative age. In the H-R Diagram, the YSO s studied are identified by the larger circle symbols. The RBSE Journal V1.6

8 Age Determination Method 2: Degree of Emissivity Stars should become less emissive as they age, because the nebulae surrounding them should dissipate and they should become more stable. Initially, in order to rank by age based on this concept, all emission lines in the spectra were reviewed and stars were ordered with the most emission lines indicating younger age. But this method could be inaccurate because these highly variable stars might have been in an outburst when they were observed, and the emission may not be representative of the base (quiescent) spectrum. In the second and final analysis, stars were only reviewed based on specific elemental lines determined to be emissivity indicators. For instance, the most emissive stars were those with dominant Fe I lines. There are several potential problems with this method. One major issue was that there is no way to determine which emission lines originate from the star and which originate from the nebula. Interstellar media may absorb some emission lines, making the star appear older. Additionally, young stars such as those studied are highly variable, and looking at spectra obtained when the star is at a particularly active or quiescent stage will not give an accurate representation of the star overall. In the spreadsheet Relative Age Comparisons and Correlations, 1 is considered the youngest for Relative Age. For Emissive Severity, 1 is considered the most emissive. Relative Age Emissive Star (H-R Corrected) Severity DG TAU 5 6 DR TAU 7 7 FU ORI HBC HBC HD R MON 14 3 RY TAU 1 13 T TAU 2 8 V0380 ORI 6 1 V1321 ORI V1331 CYG 8 5 V594 CAS 4 2 XY PER 3 14 Age Determination Method 3: Other Spectral Anomalies None these features seemed practical in determining a relative age. Observing the deviation of the Balmer decrement could measure the degree of obscuration, but it is not necessarily true that the youngest stars will always be the most obscured or vice versa. Observing inflows and outflows was not very effective because they were not present in many stars. Observing changes in nebulosity in the stars over a period of time was also not very effective, due to the small amount of data to evaluate. No observed changes were seen. As for observing such features as bow shocks, jets, and the presence of The RBSE Journal V1.6

9 accretion discs, many stars lacked all of these features. Some of them may in fact have discs or jets but the geometry of the star relative to Earth or the intervening dust may prevent their detection. Other observed correlations Although many types of data were tested for correlations, only one was found. There is a linear correlation between the degree of emissivity versus the absolute magnitude. As a star becomes less emissive, it becomes brighter. This can mean two things: more massive stars produce less emission lines, or that a star is less emissive because the gas and dust around it dissipates, so one can see more of the star, and thus it brightens. The RBSE Journal V1.6

10 SUMMARY AND CONCLUSIONS The H-R diagram method for determining the relative age of the stars seemed to make the most sense, due to the plotted stars agreement with the position of the Hayashi tracks. This data also relied on photometric data that was de-reddened, which compensates for obscuring gas and dust. When plotting the degree of emissivity versus the absolute magnitude, a linear correlation is found. This means that as a star becomes less emissive, it becomes brighter. This can mean two things: that more massive stars emit less, or that as a star is less emissive, the gas and dust around it dissipates, so one can see more of the star, and thus it brightens. ACKNOWLEDGEMENTS Special thanks to our teacher Ardis Herrold, Dr. Steve Howell, Julie Krugler, NOAO staff, and Lynn and Mike Rice of New Mexico Skies for their assistance, and the use of their facilities. Visit the full paper webpage at < The RBSE Journal V1.6

11 REFERENCES Aladin Sky Atlas. Centre de Données Astronomiques de Strasbourg. < aladin.ustrasbg.fr/aladin.gml>. American Association of Variable Star Observers. American Association of Variable Star Observers. < Brinceño, C. and A.K. Vivas and J. Hernández. McNeil s Nebula in Orion: The Outburst History. Astronomy and Astrophysics. 1 Apr Cohen, Martin and Leonard V. Kuhi. A Remarkable Structural Change in a Faint Cometary Nebula. The Astronomical Journal. 28 Mar 1979: L127. Cohen, Martin. Red and Nebulous Objects in Dark Clouds: A Survey. The Astronomical Journal. 27 Aug 1979: 29. ESA FITS Liberator Site Step-by-step guide to making your own images < Hewitt, Nick. Observing Variable Nebulae. The BAA Observer Workshops. 15 Feb 2005: 357. Jaschek, Carlos, and C Sterken. The Light Curves of Variable Stars. Cambridge, England: Cambridge University Press, Jaschek, Carlos, and Mercedes Jaschek. The Behavior of Chemical Elements in Stars Cambridge, England: Cambridge University Press, Jaschek, Carlos, and Mercedes Jaschek. The Classification of Stars Cambridge, England: Cambridge University Press, Kun, M. and others. Optical and Near Infrared Observations of V1647 Ori and McNeil s Nebula in February--April Astronomy And Astrophysics. 24 Aug Levreault, Russel M. Interactions Between Pre-Main-Sequence Objects and Molecular Clouds. II. PV Cepei. The Astrophysical Journal. 28 Jul 1983: 634. Reipurth, Bo. A General Catalogue of Herbig-Haro Objects. Center for Astrophysics and Space Astronomy SIMBAD Astronomical Database. Centre de Données Astronomiques de Strasbourg. < The Balmer Decrement < VizieR. Centre de Données Astronomiques de Strasbourg. < The RBSE Journal V1.6