molecular interstellar absorption toward the pleiades star cluster adam ritchey department of...
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Molecular Interstellar Absorption toward the Pleiades Star Cluster
Adam Ritchey
Department of Physics & Astronomy
University of ToledoJune 21, 2006
Outline of Talk• Introduction to the Pleiades
– Significance to interstellar studies– Motivation for the present investigation
• Observing Program
• Data Reduction and Fitting Procedure
• Results and Analysis– Determination of column densities– Implications of average velocities and b-values– Velocity structure of molecular gas– Physical conditions from models of diffuse cloud chemistry
• Acknowledgements
Introduction to the Pleiades
• Laboratory for the study of interstellar processes due to the interaction between the stellar radiation field of the cluster and surrounding diffuse interstellar clouds.
• Spatial association of stars and ambient interstellar medium (ISM) is ephemeral – result of a chance encounter between the cluster and one or more approaching clouds (White 2003).
Blue image of the Pleiades from the Digitized Sky Survey
Introduction to the Pleiades• Anomalous characteristics:
– Unusually strong CH+ absorption (Adams 1949).
– High degree of H2 rotational excitation (Spitzer et al. 1974).
• Previous Investigations:– White (1984) examined CN, Ca II,
Ca I, CH+, and CH toward 15 Pleiades members. (v ~ 3–8 km s-1)
– Crane et al. (1995) surveyed CH+ and CH at ultra-high resolution. (v ~ 0.6 km s-1)
– White et al. (2001) analyzed the Na I D lines of 36 stars in the Pleiades. (v ~ 1.1–1.5 km s-1) Blue image of the Pleiades from the Digitized Sky Survey
Primary motivation: to obtain equally high quality data for the other important optical tracers of the ISM toward a large number of targets in the Pleiades so that a complete picture of the interaction between interstellar gas clouds and the stars of the cluster may be constructed.
Observing Program• 20 stars from the list in White et al. (2001) with secure membership in the cluster.
• Observed using the high-resolution mode (R ~ 175,000; v ~ 1.7 km s-1) of the 2dcoude spectrograph on the Smith 2.7 m telescope at McDonald Observatory.
• Instrumental setup allowed for the detection of absorption features from CN 3874, Ca II K 3933, Ca I 4226, CH+ 4232, and CH 4300.
Smith 2.7 m Telescope
(Ritchey et al. 2006)
Data Reduction and Fitting Procedure• Standard IRAF routines were used to extract 1-D spectra that were Doppler-corrected and
normalized to unity.
• Signal-to-noise ratio (SNR) typically ~ 100–200.
• Gaussian fits yielded the equivalent width (W),
radial velocity (vLSR), and Doppler parameter
(b-value) for each component.
• Component structure was constrained by the
derived b-values (0.4 km s-1 < b < 2.5 km s-1)
and velocities (compared to Ca II).
(Pan et al. 2005)
Ca II K Absorption Lines
(Ritchey et al. 2006)
• Multiple components of Ca II K detected along all twenty sight lines.
• Strongest absorption at vLSR ~ 7 km s-1.
CH+ Absorption Lines
(Ritchey et al. 2006)
• CH+ just as pervasive yet only one or two components per sight line.
• (Note the stellar feature toward HD 23964.)
CH+ Absorption Lines
(Ritchey et al. 2006)
• CH+ just as pervasive yet only one or two components per sight line.
• (Note the stellar feature toward HD 23964.)
Stellar
CH Absorption Lines
(Ritchey et al. 2006)
• Marginal detections of CH toward five stars. Much stronger component toward HD 23512.
• (Note, again, the stellar features toward HD 23964.)
CH Absorption Lines
(Ritchey et al. 2006)
• Marginal detections of CH toward five stars. Much stronger component toward HD 23512.
• (Note, again, the stellar features toward HD 23964.)
Stellar
CH Absorption Lines
(Ritchey et al. 2006)
• Marginal detections of CH toward five stars. Much stronger component toward HD 23512.
• (Note, again, the stellar features toward HD 23964.)
HD 23512
Stellar
The Sight Line toward HD 23512• This sight line passes through a small molecular cloud first mapped in CO emission by Cohen
(1975, unpublished; see Federman & Willson 1984).
• Our velocities for the CH and CN components toward this star (+9.8 and +9.4 km s -1) agree with the value determined from the radio data (~10 km s-1).
(Federman & Willson 1984)
CN Absorption
(Ritchey et al. 2006)
CO Emission:
The Sight Line toward HD 23512• This sight line passes through a small molecular cloud first mapped in CO emission by Cohen
(1975, unpublished; see Federman & Willson 1984).
• Our velocities for the CH and CN components toward this star (+9.8 and +9.4 km s -1) agree with the value determined from the radio data (~10 km s-1).
(Federman & Willson 1984)
CN Absorption
(Ritchey et al. 2006)
HD 23512
HD 23512
CO Emission:
• Column densities (N) interpolated from curves of growth based on the measured equivalent widths.
• Adopted average b-values: 1.6 km s-1 for Ca II, CH+, and CH, 0.5 km s-1 for CN.
• (Since most lines are weak, the use of different b-values does not impact the derived column densities in any appreciable way.)
Ca II K
CN R(0)CN R(1)
CN P(1)
CH+
CH
Curves of Growth for Various Species
Determination of Column Densities
• Column densities (N) interpolated from curves of growth based on the measured equivalent widths.
• Adopted average b-values: 1.6 km s-1 for Ca II, CH+, and CH, 0.5 km s-1 for CN.
• (Since most lines are weak, the use of different b-values does not impact the derived column densities in any appreciable way.)
Ca II K
CN R(0)CN R(1)
CN P(1)
CH+
CH
Curves of Growth for Various Species
linear
Determination of Column Densities
(Ritchey et al. 2006)
Average Velocities and b-values
(Ritchey et al. 2006)
• We find a kinematic distinction between atomic and molecular gas, with atomic absorption occurring near 6 or 7 km s-1 and molecular absorption near 9 km s-1.
Average Velocities and b-values
• We find a kinematic distinction between atomic and molecular gas, with atomic absorption occurring near 6 or 7 km s-1 and molecular absorption near 9 km s-1.
• Mean b-values for the molecular species indicate that toward most of the Pleiades (except the sight line toward HD 23512) CH is associated with CH+ rather than CN.
(Ritchey et al. 2006)
Average Velocities and b-values
• For molecular species, one component was typically found per absorbing sight line, falling in either of two categories:
– Weak components with vLSR ~ +7 km s-1
– Stronger components with vLSR ~ +9.5 km s-1
• The two components generally occupy different regions of the cluster.
(Ritchey et al. 2006)
Velocity Structure of Molecular Gas
• For molecular species, one component was typically found per absorbing sight line, falling in either of two categories:
– Weak components with vLSR ~ +7 km s-1
– Stronger components with vLSR ~ +9.5 km s-1
• The two components generally occupy different regions of the cluster.
(Ritchey et al. 2006)
CH:
7 km s-1
9.5 km s-1
Velocity Structure of Molecular Gas
• For molecular species, one component was typically found per absorbing sight line, falling in either of two categories:
– Weak components with vLSR ~ +7 km s-1
– Stronger components with vLSR ~ +9.5 km s-1
• The two components generally occupy different regions of the cluster.
(Ritchey et al. 2006)
CH: CH+:
7 km s-1
7 km s-1
9.5 km s-1 9.5 km s-1
Velocity Structure of Molecular Gas
• We derived estimates for the total gas density, n, and the intensity of the incident UV radiation field, Iuv, by adopting various models of diffuse cloud chemistry.
Physical Conditions of the ISM near the Pleiades
• We derived estimates for the total gas density, n, and the intensity of the incident UV radiation field, Iuv, by adopting various models of diffuse cloud chemistry.
Steady-state model of CH formation from CH+:
The rate equation for CH may be written: (Welty et al. 2006)
2Hor HCHeCH
and H,CHHCH
H,CHHCH
2-
3
322
22
.)CH(
)H()CH()H,CH(67.0)CH( 22
GI
nfNkN
uv
Physical Conditions of the ISM near the Pleiades
• We derived estimates for the total gas density, n, and the intensity of the incident UV radiation field, Iuv, by adopting various models of diffuse cloud chemistry.
Steady-state model of CH formation from CH+:
The rate equation for CH may be written: (Welty et al. 2006)
2Hor HCHeCH
and H,CHHCH
H,CHHCH
2-
3
322
22
(1) )H()H,CH(67.0
)CH(
)CH(
)CH(
22 fk
GI
N
Nn uv
.)CH(
)H()CH()H,CH(67.0)CH( 22
GI
nfNkN
uv
Physical Conditions of the ISM near the Pleiades
• We derived estimates for the total gas density, n, and the intensity of the incident UV radiation field, Iuv, by adopting various models of diffuse cloud chemistry.
Steady-state model of CH formation from CH+:
The rate equation for CH may be written: (Welty et al. 2006)
2Hor HCHeCH
and H,CHHCH
H,CHHCH
2-
3
322
22
(1) )H()H,CH(67.0
)CH(
)CH(
)CH(
22 fk
GI
N
Nn uv
.)CH(
)H()CH()H,CH(67.0)CH( 22
GI
nfNkN
uv
fn
1
Physical Conditions of the ISM near the Pleiades
Optical Pumping model of H2 rotational levels: J = 4 and 5 levels populated primarily by photon pumping. In environments of low to moderate density, these levels will be depopulated by spontaneous emission. The relative populations of higher and lower rotational levels indicates the density of the gas: (Lee et al. 2002)
).H(/)H(2)H(2) H()H( where
,19.0)1()0( 11.0
)0(26.0
)H(
)4(102.9
222tot
1
tot
7
I fNNNN
NN
N
N
Nn
Physical Conditions of the ISM near the Pleiades
Optical Pumping model of H2 rotational levels: J = 4 and 5 levels populated primarily by photon pumping. In environments of low to moderate density, these levels will be depopulated by spontaneous emission. The relative populations of higher and lower rotational levels indicates the density of the gas: (Lee et al. 2002)
).H(/)H(2)H(2) H()H( where
,19.0)1()0( 11.0
)0(26.0
)H(
)4(102.9
222tot
1
tot
7
I fNNNN
NN
N
N
Nn
(2) 19.0)1()0( 11.0
)0(26.0
)1()0(2
)H()4(102.9
1
27
NN
N
NN
fNn
Physical Conditions of the ISM near the Pleiades
Optical Pumping model of H2 rotational levels: J = 4 and 5 levels populated primarily by photon pumping. In environments of low to moderate density, these levels will be depopulated by spontaneous emission. The relative populations of higher and lower rotational levels indicates the density of the gas: (Lee et al. 2002)
).H(/)H(2)H(2) H()H( where
,19.0)1()0( 11.0
)0(26.0
)H(
)4(102.9
222tot
1
tot
7
I fNNNN
NN
N
N
Nn
(2) 19.0)1()0( 11.0
)0(26.0
)1()0(2
)H()4(102.9
1
27
NN
N
NN
fNn
fn
Physical Conditions of the ISM near the Pleiades
• Since eqn. (2) is proportional to f and eqn. (1) is inversely proportional to f, f can be varied as a free parameter until our density determinations agree.
• We find that f = 0.07 provides the best agreement.
• Therefore, we find the density of the ISM near the Pleiades to be n ~ 50 cm-3.
(Ritchey et al. 2006)
Results of Density Determinations
• Since eqn. (2) is proportional to f and eqn. (1) is inversely proportional to f, f can be varied as a free parameter until our density determinations agree.
• We find that f = 0.07 provides the best agreement.
• Therefore, we find the density of the ISM near the Pleiades to be n ~ 50 cm-3.
(Ritchey et al. 2006)
f = 0.07
Results of Density Determinations
Conclusion: with the availability of high quality data for CN, Ca II, Ca I, CH+, and CH from this study and for Na I from White et al. (2001), existing models of interstellar chemistry must now be improved and brought to bear on the complex interaction taking place between diffuse gas and the Pleiades cluster.
AcknowledgementsS. R. Federman (Univ. Toledo)
K. Pan (Apache Point Obs.)
D. L. Lambert (Univ. Texas)
M. Martinez (Univ. Wash.)
Y. Sheffer (Univ. Toledo)
D. E. Welty (Univ. Chicago)
Anonymous Referee
ReferencesAdams, W.S. 1949, ApJ, 109, 354
Crane, P., Lambert, D.L., & Sheffer, Y. 1995, ApJS, 99, 107
Federman, S.R., & Willson, R.F. 1984, ApJ, 283, 626
Lee, D.-H., Min, K.-W., Federman, S.R., Ryu, K.-S., Han, W.Y., Nam, U.-W., Chung, H.-S., Dixon, W.V.D., & Hurwitz, M. 2002, ApJ, 575, 234
Pan, K., Federman, S.R., Sheffer, Y., & Andersson, B.-G. 2005, ApJ, 633, 986
Ritchey, A.M., Martinez, M., Pan, K., Federman, S.R., & Lambert, D.L. 2006, in press
Spitzer, L., Jr., Cochran, W.D., Hirshfeld, A. 1974, ApJS, 28, 373
Welty, D.E., Federman, S.R., Gredel, R., Lambert, D.L., & Thorburn, J.A. 2006, in press
White, R.E. 1984, ApJ, 284, 685
White, R.E., 2003, ApJS, 148, 487
White, R.E., Allen, C.L., Forrester, W.B., Gonnella, A.M., & Young, K.L. 2001, ApJS, 132, 253