The 5 to 25 micron observations show that an object previously discovered at 2.2 microns (IRS 5) within the densest part of the L1551 dark cloud is a strong source of radiation from grains as cool as 230 K. The energy distribution resembles that of infrared objects embedded within other molecular cloud cores, but implies a total solar luminosity of only 30. The luminosity of the source and its proximity to other T Tauri stars suggests that it is a 1 to 2 solar mass pre-main sequence star still swaddled within the L1551 cloud. The radio and optical evidence for mass motions around IRS 5 may mean that IRS 5 has been flaring in FU Ori-type eruptions within the last 1000 years

Beichman, C.

Harris, S.

English

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https://ntrs.nasa.gov/search.jsp?R=19810002393 2020-03-21T16:30:46+00:00Z
(NASA-CA-16360) THE FORMATION OF A T TAURI
	 N81-1U901
STAR: OBSERVATIONS OF THE INFRARED SOURCE
IN L 1551 (California last. of Tech.) 19 p
HC AU2/MF A01
	 CSCL 03A
	 Unclas
G3/89 36616
THE FORMATION OF A T TAURI STAR:
OBSERVATIONS OF THE INFRARED SOURCE IN L 1551
Charles Beichman*
California Institute of Technology
Pasadena, California 91125
and
Stella Harris*
Queen Mary College
London, England
r	 *Visiting Astronomer at the Infrared Telescope Facility which is operated
by the University of Hawaii under contract from the National Aeronautics
and Space Administration.
25-25 µm observations show that the object discovered by Strom
et al. (1976) at 2.2 pm within the densest part of the L1551 dark
cloud is a strong source of radiation from grains as cool as 130 K.
The energy distribution resembles that of infrared objects embedded
within other molecular cloud cores, but implies a total luminosity of
only 30 1 0 . The luminosity of the source and its proximity to other T
Tauri stars suggests that it is a 1-2 M0 pre-main sequence star still
swaddled within the L1551 cloud.
The radio and optical evidence for mass motions around IRS 5
(Snell et al. 1980; Cudworth and Herbig 1979) may mean that IRS 5 has
been flaring in FU Ori-type eruptions within the last 10 3 years.
Subject Headings: infrared: sources - nebulae:
individual - stars: pre-main-
sequence
I It
3I. INTRODUCTION
L1551 is a nearby dark cloud in Taurus (Lynda 1962) which is
ts:,ociated with a number of T Tauri stars and Herbig-Haro objects
(figure 1) as well as with CO emission characterized by extremely
broad line profiles (Knapp It al. 1976). Strom, Strom and Vrba (1976;
hereafter denoted SSV) made 1.2-4.8 µm observations of a number of
objects in the vicinity. One source, their #5, hereafter called
L1551-IRS 5, showed an energy distribution rising steeply to wavelengths
longer than 5 pm, suggesting emission from hot dust surrounding an
object embedded within the molecular cloud. The infrared properties
of IRS 5 and its location within the dense core of the cloud (Snell
1979; Loren, Evans and Knapp 1980) are similar to those of extremely
young, late 0 or early B stars found within the cores of massive
molecular clouds such as S 140, S 255 and Cep A (Beichman, Becklin and
Wynn-Williams 1979).
A most intriguing aspect of L1551 is the evidence for large scale
mass motions within the molecular cloud apparently centered on the
position of IRS 5. Snell, Loren and Plambeck (1980) have found that
the broad wings of the CO lines arise in two streams of gas flowing in
diametrically opposite directions outward from IRS 5 (figure 1). There
is also suggestive optical evidence for the existence of mass motions.
A proper motion study of the nearby Herbig-Haro objects HH 28 and HH 29
(Cudworth and Herbig 1979) revealed that these two objects are both
moving directly away from IRS 5 at about 150 km s-1 in the same direcLion
as the southwest flow of gas seen in CO.
t3
4Snell et al. (1980) suggest that the expanding gas and the large
proper motions of the HH objects are due to the interaction of a strong
stellar wind around IRS 5 with the ambient gas of L1551. The nature of
IRS 5 is central to the discussion and in this paper we report 5-25 µm
observations of this object.
}	 II. OBSERVATIONS
x	 We observed L1551-IRS 5 on 23 October (UT), 1979 with the newly
commissioned NASA Infrared Telescope Facility atop Mauna Kea, Hawaii.
The University of Hawaii helium bolometer (Dyck and Simon 1977) was
used at the f/35 Cassegrain focus of the 3 meter telescope. The secondary
mirror was chopped at 14 Hz with a 30" throw in the north-south direction.
Scans across a Peg at 10 and 20 pm showed the focal plane aperture to
have a full-width at half-maximum of 3.8. The seeing was about 1"
during the course of the night.
Narrow-band filter photometry was calibrated with respect to
a Tau whose magnitudes shortward of 18 µm are from Neugebauer (private
communication 1979) and longward of 18 pm from Morrison and Simon
(1973). The conversion from magnitudes to flux densities is given
in Beckwith et al. (1976). The chief sources of photometric uncertainty
appear to have been due to guiding and to uncertainties in the
calibration. The latter are estimated to be 10 percent shortward of
18 pm and 20 percent longward of 18 µm.
III. DATA
The position of IRS 5 is x(1950) - 04h 28m 4052 and 6(1950)
I
5180 01' 45" with an uncertainty of 3 2" in each coordinate. The position
is derived from incremental encoder readouts of offsets from HL Tau
for which an astrometric position is available (Herbig, private communica-
tion 1979).
Data taken at positions' around the source show it to be smaller
than 4" in diameter. There is no evidence for any other source brighter
than 1 Jy at 10 pm within 15" of IRS 1.
The photometry of the source is given in Table 1 and Figure 2.
The present data overlaps that of SSV only at 4.8 } ►m wich our datum
being about a factor of two lower than theirs. The disagreement could
be due to the effects of observing a slightly extended source with
radically different beam sizes (30" vs. 3"8) or to source variability.
IV. DISCUSSION
Within the accuracy of the radio measurements IRS 5 lies projected
against the densest part of the molecular cloud. The observations of
Snell (1979) and Loren et- al. (1980) give a 13CO column density of
7.5 x 1015 cm2 and a peak density of 6.3 x 104 cm-3 . The location of
IRS 5 is consistent with its having formed as a result of the collapse
of the dense core of L1551.
The overall shape of the 1.2-25 pm energy distribution is very
similar to those of other sources embedded within molecular clouds
such as S140-IRS 1, S255-IRS 1 and IC 1848-IRS 1 (Beichman et al.
1979; Blair et al. 1978; Evans, Blair, and Beckwith 1977; Loren and
Wootten 1978). The spectrum of L1551-IRS 5 rises steeply out to 25 µm
6and is much broader than that arising from the emission of single
temperature dust grains. The 12.8-25 pm color temperature is 130 K t 5 K
for blackbody emission and 112 t 5 K for grains following a a 'l emissivity
law. The 3.4-4.8 pm data of SSV yield a blackbody color temperature of
660 K. Such a broad energy distribution, implying a large range in
grain temperature, is consistent with models of the radiative trans"Ler
through clouds heated by a central heat source (e.g. Scoville and Kwan
1976).
The ratio of the broadband 10 pm flux to the 10.5 pm narrow-band
flux is 1.2 t 0.1, suggesting the existence of a shallow silicate
absorption feature.
Despite its resemblance to infrared sources in other molecular
clouds, L1551 - IRS 5 is far less luminous than other embedded objects.
At a distance of 140 pc (Elais 1979) the 1.2-25 pm luminosity of IRS 5
is only 2.4 L, The existence of grains as cool as 130 K suggests that
much of the emission from IRS 5 comes at wavelengths longer than 2511m.
4	
A comparison with the energy distributions of embedded sources like
S 140 and S 255 suggests that as much as ten to twenty times more
energy will be emitted at far infrared wavelengths as is emitted
shortward of 25 µm. The total luminosity of IRS 5 is probably in the
range of 25-50 L.. The far infrared observations of Fridlund et al.
(1980) give a total luminosity of 25 L Q
 in a 4.5 beam.
An independent measure of the total luminosity of IRS 5 comes
from assuming that the gas temperature observed in 12 COis the result
of collisional heating of the gas by dust grains which are themselves
7warmed by radiation from IRS 5. The method for calculating far infrared
luminosities from CO data described by Evans et al. (1977) can be
combined with the 12CO and 13CO observations of Snell (1979) to yield
a lower limit of 30 L® to the far infrared luminosity of the material
within the 12 K contour of Snell's map (within 2'-3' of IRS 5). The
energy output required of IRS 5 depends on the fourth power of the assumed
dust temperature, Td , less a correction factor because the grains,
even in the absence of IRS 5, have an equilibrium temperature T eq ~
10 K due to non-localized heat sources such as cosmic rays (Goldreich
and Kwan 1974). For Td k 15 X. the correction factor, 1-(Teq/Td)41
is negligible compared with other uncertainties. The assertion that the
grains are heated by IRS 5 is supported by equation 9 of Scoville and
Kwan (1976) which shows that a 30 L0 object can heat material 2'-3'
away to temperatures of 15-20 K, comparable to the gas temperatures
observed at that radial distance from IRS 5.
An energy output of about 30 Le is characteristic of a ZAMS AO-A5
star, although the long time scale of evolution of stars of low luminosity
and their near vertical approach in the HR diagram to the main sequence
makes it dangerous to assign a spectral type on the basis of luminosity
alone. Perhaps more meaningful than a comparison with ZAMS stars
is one with visible pre-main sequence stars. T Tauri, V 380 Ori and
RY Tauri are all emission-line stars associated with dust emission
from nearby molecular clouds. Their total luminosities are in the
range 10-80 L0
 (Harvey, Thronson and Gatley 1979). IRS 5 may be an
object like these stars, or a precursor to tbem, but still so heavily
enshrouded by its parent cloud that no optical star is visible.
It is possible, however, that some of the optical radiation from
8IRS 5 is escaping from the cloud. The extended nebula HH 102 (also
F
	 known as S 239) lies a few arcminutes west of IRS 5. Recentpolarimetric
k'
	
measurements (Jrba 1980, private communication) support the suggestion
of SSV that HH 102 is a refle..tion nebula illuminated by light from
IRS 5. The spectrum of HH 102 taken by Strom, Grasdalen and Strom
(1974) is, then, one of IRS 5 which is responsible for heating the dust
observed in the infrared. HH 102., and by inference, IRS 5, show spectral
lines characteristic of a low excitation, partially ionized gas.
The spectral lines are quite similar to those seen by Strom et al.
(1974) in the spectra of the nearby T Tauri stars HL and X2 Tauri, These
two stars lie within 0.25 pc of IRS 5 and have luminosities of 10 and
40 Le, respectively. (Rydgren, Strom and Strom 1976). It seems quite
reasonable to assert that IRS 5 is a younger sibling of these stars, born
of the same cloud and in the process of becoming a 1-2 M0 T Tauri star.
Although the majority of visible T Tauri stars studied by Cohen and
Kuhi (1979) have luminosities less than 10 L®, a few have luminosities
as high as 80 L®. The difference between IRS 5 and most T Tauri stars
could be due to either its slightly greater mass or earlier evolutionary
stage. An alternative explanation for the high luminosity of IRS 5
is the suggestion by Herbig (1977) and Larson (1980) that at any one time
about 10 percent of all T Tauri stars will have luminosities in excess
of 25 LO during the decline in energy output following an FU Ori outburst.
During such erruptions, which recur sporadically every 10 3 -104 yr, the
stellar output increases a hundredfold to 103 L  and then decays over
10-1000 yr back to less than 10 L ®. The factor of two decrease in the
4.8 pm flux in the approximately five years between the Kitt Peak and
f
9Mauna Kea observations could be due to such a post-outburst decline.
The ejection of matter at high velocities is an almost inescapable
part of Larson's model for the FU Ori phenomenon and could account for the
kinematics of the streams of gas seen in CO for the large motions of
the HH objects. In this view the HH objects are the interstellar bullets
envisioned by Dopita (1978) and by Norman and Silk (1979), accelerated
by IRS 5 during periods of high luminosity.
V. CONCLUSIONS
The object found by SSV at 2.2 µm shows an energy distribution
out to 25 pm characteristic of hot dust with temperatures between 130
and 600 K. The luminosity shortward of 25 pm is 2.4 Lo and an extra-
polation to longer wavelengths implies a total luminosity of about 30 L0
consistent with far-infrared results. A star as late as AO-A5 and
perhaps similar to the T Tauri stars found nearby has formed, or is forming,
in the same manner as more massive OB stars--namely as the result of the
collapse of a dense, 10 5 cm 3 , molecular cloud core.
Accounting for the present high luminosity of IRS 5 and the mass
motions in its vicinity may require that IRS 5 have experienced a number
of FU Ori-type outbursts within the last 2000 yr.
,y.,,^..^.-...,..	 . ; ^..,.,.. n ,...,.; .r^....^.^^,^..,^f.-..r.w^.z,rr-'^'^!c.,+'e..^*g^",°^^a^.sa.^r-^ -,.w^•,..z-.^--..	 . ,,,	 ...	 ..	 _	 _
1
'	 1
10
TABLE 1
Infrared Fluxes
from
L1551 - IRS 5
71 (µO sv(JY)
4.8 0.50 t 0.07
10.5 2.3 t 0.1
10 (broadband)	 2.8 t 0.2
12.8 6.9 t 0.6
18 20 t 2
20 (broadband)
	 37 t 2
25 63 t 6
11
ACKNOWLEDGEMENTS
We would like to thank Drs. A. Sargent, F. Vrba and R. Plambeck for useful
discussions and for communicating their results in advance of publication.
Dr. G. Herbig kindly provided the astrometric positions for stars in
the vicinity of L1551. The staff of the IRTF and in particular night
assistant Ron Kohler were most helpful during the course of the observing
run and in overcoming teething problems with the new telescope. Drs. E. E.
Becklin and G. Neugebauer provided a number of useful comments.
The observations were supported in part by NASA grant NGL 05-002-207.
F
.t
12
REFERENCES
Beckwith, S., Evans, N. J., II. Backlin, E. E., Neugebauer. G. 1976,
AV. J., 208, 390.
Beichman, C. A.. Backlin. E. E.. Wynn Williams, C. G. 1979, Ap. J.
(Letters), 232, L47.
Blair, G. N.. Evans, N. J.. ii, Van din Bout, P. A.. Peters W. L, III,
1978, Ap J., 219, 896.
Cohen, M. and Kuhl, L. V. 1979, AV. J. (Letters), 2'
 27, L105.
Cudworth, K. M. and Herbig, G. H. 1979. A. J., 84, 548.
Dopita, M. 1978, Ap. J. (Suppl.). 37, 117.
Dyck, H. M. and Simon, T. 1977, Ap. J., 211. 421.
Elias. J. H. 1979, Ap. J., 224, 857.
Evans, N. J.. II, Blair, G. N., Beckwith, S. 1977, &. J.. 217. 448.
Fridlund, C. V.. Nordh, H. L., Van Duinen, R. J.. Aalders, T. W. G..
Sargent. A. I. 1980, Astr. Ap., in press.
Goldreich, P. and Kwan J. 1974, Ap. J., 189, 441.
Harvey, P. M.. Thronscnt, H. A.. Gatley. I. 1979,	 J., 231, 115.
Herbig. G. H. 1977, Ap. J., 217, 693.
Knapp, G. R., Kuiper, T. B. H.. Knapp, S. L., Brown, R. L. 1976,
Ate.. 206. 443.
Larson, R. B. 1980, M.N.A A.S.. 190, 321.
Loren, R. B., Evans, N. J., II, Knapp. G. R. 1979. Ap. J., 234. 932.
Loren, R. B. and Wootten, A. H. 1978. ho. J. (Letters 	 225. L81.
Lynda, B. T. 1962. &. J. (Supple). 7, 1.
Morrison. D. and Simon, T. 1973. Ap. J., 186, 193.
Norman, C. and Silk, J. 1979, Ap. J. 228. 197.
13
Rydgren, A. S., Strom, S. E., Strom, K. M. 1976, Ape J. (SuUI.)
30, 307.
Scoville, N. J. and Kwan. J. 1976, Ape J. 2206, 718.
Snell, R. L., Loren, R. Be, Plambeck, R. L. 1980, preprint.
Snell, R. L. 1979, Ph.D. thesis, Univ. of Texas, NSF TR AST
77-28475-79-1.
Strom, K. M., Strom, S. 8., Vrba, F. J. 1976, A,_,J., 81, 320. (SSV).
Strom, S. K., Crasdalen, C. L., Strom, K. M. 1974, .-.9. J., 191, 111.
;.	 14
k
FIGURE CAPTIONS
Figure 1.
	
A schematic drawing of the region ar6tind L1551-IRS 5 with
-north up and east to the left. IRS 5 is denoted by a
large filled circle. The Herbig-Harp objects HH 28 and 29
and the Tauri stars HL Tau'and XZ Tau are shown as small
filled circles. The extended nebula HH 102 is denoted
by the hatched area west-of IRS 5.
r
The lines joining IRS 5, HH 28 and HH 29 approximate the
direction of motions inferred from the proper motio ►is:
i.
observed by Cudworth and Herbig (1979). As these authors
is
point out, if the proper motions are interpreted as real
space velocities, then HH 28 and HH 29 were in the vicinity
of IRS 5, 2000 and 600 years ago, respectively.
The dashed contours show the extent of the CO streams observed
by Snell et al. (1980). HH 28, HH 29 and the southwest
stream of gas are all moving radially toward the observer
(Snell et al. , 1980; Strom et al., 1974).
Figure 2.	 The photometry from Mauna Kea circles is combined with that
from SSV (triangles). One sigma errors are shown on the
Mauna Kea data if these exceed the size of tine filled 	 y
i
circles.
It
15
ADDRESSES OF THE AUTHORS
CHARLES A BEICHMM
Division of Physics.
Mathematics and Astronomy
Downs Laboratory 320-47 -
California Institute of Technology
Pasadena, CA 91125
STELLA HARRIS
Department of Physics
Queen Mary College
Mile End Road
London, E1 4NS
GREAT BRITAIN
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I


The formation of a T Tauri star: Observations of the infrared source in L 1551

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