Download Methods of Determining Relative Age of Young Stellar Objects

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
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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 Stars1. 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)
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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 studies2 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, 1990.
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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.
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Spectroscopy
Spectral
Star
Type
DG TAU
G0Ve
DR TAU
F0V
FU ORI
Spectral Features
Star Type
Emission
Lines
Blue
Ca II; Fe I, II; O
III
Ca II; Fe I, II; O
III
Red
Effective
Temp
(K)
NA
5920
NA
7240
NA
5730
HAeBe
Ca II; O III
Ca II; Fe I, II; O
III
NA
30000
HAeBe
Ca II; O III
Ca II
17600
DP
HAeBe
Ca II; Fe II; O III
Ca II
15400
BS; Jet*
T Tau
G4V
PI
FU Ori
HBC 243
B0Ve
PC= 243, 308km/s
HBC 730
B4Ve
HD 200775
B6Ve
R MON
F5Ve
INA
Ca II; Fe II, O III
NA
6540
RY TAU
K1V
PI
T Tau
NA
5010
T TAU
G4Ve
BS
T Tau
Ca II
5730
V0380 ORI
G0Ve
INAT
NA
5920
V594 CAS
B6Ve
DP
PC = 185, 144km/s; DP;
Jet
No
Ca II; Fe I, II; O
III
Ca II; Fe I, II; O
III
Ca II; Fe I, II; O
III
Ca II
15400
V1321 ORI
G8V*
NA
5490
V1331 CYG
K0Ve
PC = 247, 185km/s; BS
T Tau
Ca II; Fe II; O III
Ca II; Fe I, II; O
III
Ca II
5240
XY PER
A5V
DP, PI
HAeBe
No
Ca II
8620
Photometry
YY Ori
HAeBe
T Tau
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
11.2
12.4
13.4
1.0
1.2
2.2
2.50
HBC 243
14.0
11.9
12.9
1.0
-2.1
-1.1
-0.02
HBC 730
9.8
10.0
10.9
0.9
0.2
1.1
0.53
HD 200775
10.8
10.8
11.9
1.1
0.0
1.1
0.40
R MON
9.4
9.4
9.2
-0.2
0.0
-0.2
1.86
RY TAU
9.8
10.7
12.5
1.8
0.9
2.7
2.68
T TAU*
8.4
8.8
10.4
1.6
0.4
2.0
2.86
V0380 ORI
11.8
11.3
12.5
1.2
-0.5
0.7
3.14
V594 CAS
9.6
9.1
10.4
1.3
-0.5
0.8
0.78
V1321 ORI*
NA
NA
NA
NA
NA
NA
0.78
V1331 CYG
NA
NA
NA
NA
NA
NA
2.01
XY PER
11.2
11.3
12.8
1.5
0.1
1.6
1.15
Balmer Decrement
Equivalent
Jacoby
Stars
Width
Equivalent Width
% Deviation
From
Jacoby
DG TAU
6.1
12.62
48.0
DR TAU
0.4
17.62
2.1
FU ORI
-3.0
15.90
-18.9
HBC 243
27.0
14.28
189.1
HBC 730
-13.8
17.84
-77.4
HD 200775
-15.9
31.74
-50.1
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R MON
3.3
12.91
25.8
RY TAU
T TAU
-5.3
34.64
-15.3
2.3
15.90
14.2
V0380 ORI
11.6
12.62
92.1
V594 CAS
0.8
31.74
2.6
V1321 ORI
NA
13.23
NA
V1331 CYG
21.7
28.62
76.0
XY PER
-105.6
21.29
-496.0
RY Tau, a star with a reddish nebula and a
fine example of photo ionization
V594 CAS, a star with a reflective nebula and
jets
HBC 730, a star completely embedded in a
reflection nebula
XY Per, a star with a white nebula and a
prime
example
of photo ionization
INSERT
SPREADSHEET
HERE
INSERT YSO PICS HERE
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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.
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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.
Star
DG TAU
DR TAU
FU ORI
HBC 243
HBC 730
HD 200775
R MON
RY TAU
T TAU
V0380 ORI
V1321 ORI
V1331
CYG
V594 CAS
XY PER
Relative Age
(H-R Corrected)
5
7
13
11
12
9
14
1
2
6
10
Emissive
Severity
6
7
11
4
9
10
3
13
8
1
12
8
4
3
5
2
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
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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.
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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
<http://staff.gpschools.org/maciola/webpages/studentprojects.htm>
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