I have performed an analysis of the UV extinction by dust along the line of sight to the young open cluster Tr 16. The observed curves are parameterized in order to extract quantitative information about the structure of the curves. Furthermore, by constructing differential extinction curves, obtained by differencing curves for stars which lie within a few arc seconds of each other on the sky, I was able to obtain a curve which is free of the effects of foreground extinction, and represents the extinction by the dust in the Tr 16 molecular cloud. I then show that this curve is nearly identical to one due to dust in the Orion molecular cloud. This result shows that dust in the Carina arm exhibits the same behavior as that in the local arm

Massa, Derck

English

NASA Technical Reports Server

Final Report for NASA Grant NAS5-31832
Title:
PI:
Institution:
, ppl/dd.. U o.
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UV Extinction Properties of Carina Nebular Dust
Derck Massa
Applied Research Corporation
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1.0 Introduction
Trumpler 16 is a very young open cluster which contains ultra-luminous object rl Car and several
03 stars. The cluster gas exhibits peculiar motions of more than 500 km/s (see, Walborn et al
1984), and the dust has a large and variable R(V) = A(V)/E(B-V). In this report, I present UV
observations of the Tr 16 dust, demonstrate systematic changes in the extinction curves which
occur as one samples deeper into the cloud, and isolate the signature of the UV extinction of the
dust within the molecular cloud.
2.0 Observations
Table 1 lists the program stars which have IUE observations. These were selected to lie in groups
within a few arc minutes of each other on the sky. The names given in Table I are from The et al
(1980). The spectral types given in parenthesis are from classifications of the low dispersion IUE
spectra. Some of the spectra were difficult to classify because the interstellar lines were broad
enough to contaminate some important stellar lines, such as Si HI _.1300. The spectra were
matched to unreddened standards to produce extinction curves (see Fitzpatrick and Massa 1990,
FM).
The extinction curves for the program stars are shown in Figure 1. These were derived exactly as
described by FM. Available IR photometry (Tapia et al 1988) is included and the curves are
compared to a family of CCM (Cardelli et al 1989) curves, with R(V) = 2, 3, 4, and 5.
3.0 Analysis
Figure 2 shows FM fits to the UV portions of the curves, where the fitting function is given by
k(x - _) = c I + c2x + c3D(T, x o ;x) + c4F(x ) ,
where x =/t-i, D(y, Xo;X ) is a Drude profile, and F(x) is a function which is zero for
x < 5.9,u -_ (see FM for details). Table 2 lists the parameters derived from the fitting procedure.
Relationships among the various observables are shown in Figure 3 Figure 3a shows the
relationship between lit extinction and E(B-V). This relationship shows that the value of R
increases as more and more of the dust associated with the cloud affects the line of sight. This
same effect was already discussed by Tapia et al. Figure 3b shows an apparent relationship
between the bump position, Xo,and E(B-V). However, this may be due to a bias in the fitting
routine. This is because, as Figure 3c demonstrates, the slope of the UV extinction decreases with
E(B-V). This relationship could influence the nonlinear fit of x0. Figure 3d shows the
relationship between the offset and slope of the UV fit, c, and c2, for the cluster stars (crosses),
compared to the same parameters derived from a diverse sample of stars studied by Fitzpatrick and
Massa (1988) (triangles). Notice that the cluster stars seem to lie systematically beneath the line
derived for the Fitzpatrick and Massa sample.
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To isolate the Tr 16 molecular cloud extinction, and to eliminate the foreground extinction,
differential extinction curves were formed by using the differences between the color excesses of
heavily reddened stars and tightly reddened stars within 2' on the sky of them. All of the stellar
pairs with AE(B-V) > 0.1 mag were summed to form a mean Tr 16 molecular cloud extinction
_lrve.
Figure 4 shows the resulting differential extinction curve for Tr 16 dust. This curve is affected
only by dust within the Tr 16 star-forming cloud. The pairs of stars used to produce the curve are:
Tr16-14&13, 15&13, 17&94, 21&20, 7&9, 8&9, and 73&115. The only heavily reddened star
not included was Tr16-76. Unlike the other cluster stars, this object has a relatively high FUV
extinction. The mean differential curve is compared to CCM curves with R = 2, 3, 4, 5, and 6.
Finally, Figure 5 compares the differential curve for Trl6 with that of the Orion star NU Ori given
by FM. The 2 curves are essentially indistinguishable.
4.0 Conclusion
The extinction curve for dust inside the Tr 16 molecular cloud closely resembles a CCM R = 5
curve, and is almost idential to that of the Orion molecular cloud, as sampled by NU Ori. The NU
Ori curve does not have to be corrected for foreground extinction because it is very small along its
line of sight compared to the extinction caused by the Orion cloud itself. In conclusion, the
observations shows that dust in another spiral arm demonstrates the same morphological behavior
as dust in the local clouds used to derive the CCM relationships. This finding was n presented at
the 181-st meeting of the AAS in Phoenix, and is currently being incorporated into a larger paper
in collaboration with Dr. Jason Cardelli at the University of Wisconsin. Cardelli has obtained IUE
observations of main sequence B and late O stars in the young cluster NGC 6611, and has come to
similar conclusions. We have decided, therefore, to combine the observations of the 2 clusters into
a single presentation.
References
Cardelli, J.A., Clayton, G.C. and Mathis, J.S. 1989 ApJ 345, 245
Fitzpatrick, E.L. and Massa, D. 1988 ApJ 328, 734
Fitzpatrick, E.L. and Massa, D. 1990 ApJS 72, 163
Massa, D. and Savage, B.D. 1984 ApJ 279, 310
Tapia, M., Roth, M., Marraco, H. and Ruiz, M.T. 1988, MNRAS 232, 661
The, P.S., Bakker, 1L and Antalova, A. 1980, A&AS, 41, 93
Walborn, N.R., Heckathorn, J.N., and Hesser, J.E. 1984 ApJ 276, 524
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Table 1: Program Stars
Name Sp Ty V B-V E(B-V) UV Match
Trl6- 8 B1.5Vb 10.90 0.14 0.42 130.5 V
Tr16-13 (B1) 10.76 0.22 0.50 B0.5 V
Tr16-14 (O-B0) 11.50 0.42 0.72 09.5 V
Tr16-15 030.5) 11.28 0.41 0.69 BO.5 V
Tr16-17 030) 11.01 0.24 0.54 B0 V
Tr16-20 BI V 10.20 0.08 0.34 B1 V
Tr16-21 030) 10.93 0.47 0.77 B0 V
Tr 16-22 (07) 11.01 0.49 0.81 07 V
Tr16-23 07 Vn 9.97 0.38 0.70 07 V
Tr16-24 031) 11.58 0.16 0.42 B1 V
Tr16-27 031) 11.06 0.14 0.40 B1 V
Tr16-33 032) 11.83 0.27 0.51 B2 V
Tr16-73 031) 11.90 0.42 0.68 B1 V
Tr16-74 031) 11.70 0.27 0.53 B1 V
Tr16-76 030) 11.19 0.42 0.70 B0.5 V
Tr16-94 B1 Vn 9.86 0.14 0.40 B1 V
Trl6-115 09 V 10.15 0.16 0.46 09.5 V
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Table 2: Fit Parameters
3
Star Match xo ? c_ q c, c4
Trl6- 8 B0.5 V 4.601 0.882 0.057 0.523 2.344 0.301
Tr16-13 B0.5 V 4.570 1.066 -0.150 0.601 2.932 0.140
Tr 16-14 O9.5V 4.585 1.193 0.036 0.507 2.971 0.155
Tr16-15 B0.5 V 4.589 1.024 0.428 0.427 2.556 0.285
Tr16-17 B0 V 4.607 1.000 0.042 0.553 1.979 0.456
Tr16-20 B1 V 4.597 0.864 0.391 0.600 2.533 0.354
Tr16-21 B0 V 4.599 0.926 0.941 0.311 2.058 0.160
Tr16-22 07 V 4.596 1.017 0.647 0.338 2.589 0.166
Tr16-23 07 V 4.567 0.806 0.531 0.467 1.554 0.169
Tr16-24 B1 V 4.604 0.974 0.290 0.511 2.362 0.354
Tr16-27 B1 V 4.594 0.881 -0.631 0.632 2.711 0.340
Tr16-33 B2 V 4.598 0.891 0.396 0.511 2.348 0.288
Tr16-73 B1 V 4.588 0.855 0.366 0.435 2.120 0.306
Tr16-74 BI V 4.580 0.793 0.188 0.466 1.791 0.246
Tr16-76 B0.5 V 4.572 0.957 0.132 0.606 2.565 0.435
Tr16-94 B1 V 4.591 0.813 -0.670 0.846 1.793 0.897
Trl6-115 O9.5V 4.583 0.909 -0.034 0.579 2.228 0.367
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Report Documentation Page
I. ReDc_rt .No.
I 2. Go_,emmenl ACC:eSS_n No.
4, l_Ue aria Suout*e
UV Extinction Properties of Carlna Nebular Dust
3. Reom4nc's Catam9 ._o.
S. Renew( Oam
February 4 t 1993
4. Pt_o,mmg O,_a,_ItNm Co_e
7. A,Ju'latt II
Dr. Derck Massa
9. P_orm,-90_;aniu_on Name _ A4_'m
Applied Research Corporation
8201 Corporate Drive, Suite 1120
Landover, MD 20785
12. $Oonsormg A;en_ Name an4 Acate$.l
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
L Pt_rnm_ Organ_u_ ReDo_ NO.
R93-205
_0. Work Uric No.
11. C.._nua¢7. or Gtimt ,'WS.
NAS_-_I83_
t 13. Tyge of ReOo_ _o ?_n_ _:)vefe4
Final
Jan 9/91 - Feb 8/93
Code 684
15. SucD_emenLIry Notes
16. AosltaCt
I have performed an analysis of the UV extinction by dust along the llne of
sight to the young open cluster Tr 16. The observed curves are parameterized
in order to extract quantitative information about the structure of the curves.
Furthermore, by constructing differential extinction curves, obtained by
differencing curves for stars which lie within a few arc seconds of each other
on the sky, I was able to obtain a curve which is free of the effects of
foreground extinction, and represents the extinction by the dust in the Tr 16
molecular cloud. I then show that this curve is nearly identical to one due
to dust in the Orion molecular cloud. This result shows that dust in the Carina
arm exhibits the same behaviour as that in the local arm.
_. Key Woras (Suggestea by Aumorlsll
interstellar dust,
UV extinction
_. Sec_ry C_s_l. (of ms rel_rt!
NASA FORM 1_lOCT N
18. O;su,butmn Statement
UNCLASSIFIED - UNLIMITED
.'0.Sec_riW q_Js._|. Iof mrs p&gel
21. No. o! _ges I _,. P_e
16 + Rpt. |\
Documentatio_
ra e ! -.


UV extinction properties of carina nebular dust

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