The Astrophysical Journal, 583:330–333, 2003 January 20 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
VERY LARGE ARRAY OBSERVATIONS OF PROPER MOTIONS IN L1551 IRS 5 Luis F. Rodrı´guez Instituto de Astronomı´a, Campus UNAM, Morelia, Michoaca´n 58190, Me´xico;
[email protected]
Salvador Curiel and Jorge Canto´ Instituto de Astronomı´a, UNAM, Apartado Postal 70-264, Me´xico, DF 04510, Me´xico;
[email protected]
Laurent Loinard Instituto de Astronomı´a, Campus UNAM, Morelia, Michoaca´n 58190, Me´xico;
[email protected]
Alejandro C. Raga Instituto de Ciencias Nucleares, UNAM, Apartado Postal 70-543, Me´xico, DF 04510, Me´xico;
[email protected]
and Jose´ M. Torrelles Institut d’Estudis Espacials de Catalunya (IEEC/CSIC) and Instituto de Ciencias del Espacio (CSIC), 08034 Barcelona, Spain;
[email protected] Received 2002 September 3; accepted 2002 September 23
ABSTRACT Using high angular resolution (0>1) Very Large Array observations made at 2 cm during the period 1983 to 1998, we report the detection of proper motions in the components of the binary radio source in L1551 IRS 5. The absolute proper motions observed in these two protostars, of order 25 mas yr1 or 17 km s1 at a distance of 140 pc, are very similar in magnitude and direction to those of T Tauri stars in the same region and are attributed to the large-scale motion of the parent molecular complex. The relative astrometry between the two components reveals orbital proper motions that suggest that the total mass and period of the binary system are 1.2 M and 260 yr, respectively. Subject headings: astrometry — ISM: jets and outflows — stars: individual (L1551 IRS 5)
ity to firmly detect the radio binary system. Unfortunately, in two of these epochs (1987.60 and 1988.88) the southern component of L1551 IRS 5 was undergoing a major ejection event and showed considerable elongation in the E-W direction. As a result of this morphology, it was not possible to determine reliably the position of the southern component and we did not use the data from epochs 1987.60 and 1988.88. The remaining four observations were made in the epochs and using the phase calibrators listed in Table 1. The observations were made in both left and right circular polarizations with an effective bandwidth of 100 MHz. The data were edited, calibrated, and imaged using the software package Astronomical Image Processing System (AIPS). We produced cleaned images of the fields setting the ROBUST parameter of the AIPS task IMAGR to 0, to optimize the tradeoff between angular resolution and sensitivity. The synthesized beam dimensions and rms sensitivity of the images, as well as the total flux density of L1551 IRS 5, are listed in Table 1.
1. INTRODUCTION
L1551 IRS 5 is a prototypical young stellar source. In the centimeter radio continuum it has been known to be double since the pioneering observations of Bieging & Cohen (1985). From 7 mm Very Large Arrary (VLA) observations made at high angular resolution (50 mas), Rodrı´guez et al. (1998) showed that at this wavelength the emission comes from two compact protoplanetary disks with projected separation of 0>3 (40 AU at a distance of 140 pc). Rodrı´guez et al. (1998) also showed that at 2 cm we are detecting a combination of dust emission from the disks and free-free emission from ionized ejecta. In this paper we analyze VLA observations made at 2 cm in the A configuration at four epochs from 1983 to 1998. These observations have an angular resolution of 0>1 and resolve the two components of the L1551 IRS 5 system, allowing both absolute and relative astrometry to be performed.
2. OBSERVATIONS AND DISCUSSION
2.1. Absolute Proper Motions
The 2 cm observations were made using the VLA at the National Radio Astronomy Observatory (NRAO)1 in the A configuration during several epochs. There are several 2 cm observations of L1551 IRS5 made over the years with the VLA in the A configuration. After a preliminary analysis, we found that six of the observations had sufficient sensitiv-
The search for absolute proper motions in the double radio source was made using the three epochs, 1983.89, 1995.57, and 1998.41, that were observed using the same phase calibrator (0400+258). We searched unsuccessfully for a compact background source in the field of the primary beam that could have allowed an absolute alignment of epoch 1985.01. In Figure 1 we show the images for the first and last epochs, with the north (N) and south (S) components of L1551 IRS 5 identified. The absolute proper motion of the sources to the southeast is evident.
1 NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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TABLE 1 VLA-A Observations of L1551 IRS 5 at 2 cm
Epoch
Phase Calibrator
Synthesized Beama
rms (mJy)
S (mJy)b
1983 Nov 20 (1983.89)..... 1985 Jan 05 (1985.01) ...... 1995 Jul 28 (1995.57) ....... 1998 May 31 (1998.41) ....
0400+258 0500+019 0400+258 0400+258
0.13 0.12;63 0.12 0.12; + 25 0.12 0.12;55 0.12 0.12;63
0.10 0.09 0.08 0.07
3.6 2.8 3.3 2.7
a b
Major axis minor axis (arcsec); position angle of major axis (deg). Total flux density at 2 cm. The error is estimated to be 0.2 mJy.
We made a least-squares linear fit to positions from the three epochs, and the results are presented in Figure 2 and Table 2. The positions of the components were obtained from quadratic fits to the images made with the MAXFIT task in AIPS. The total proper motions are on the order of 25 mas yr1, which at a distance of 140 pc implies a velocity in the plane of the sky of 17 km s1. Very similar absolute proper motions (l ¼ þ11:6 2:0 mas yr1, l ¼ 22:8 1:9 mas yr1) were obtained by Jones & Herbig (1979) for the average of six T Tauri stars (their group III) that appear projected in the sky close to the L1551 dark cloud. Recently, absolute proper motions of heavily obscured protostars in IRAS 162932422 (Loinard 2002) and IRAS 04368+2557 (Loinard et al. 2002) have been reported. As in L1551 IRS 5, the absolute proper motions of these sources are similar to those of nearby T Tauri stars. Following Loinard (2002) and Loinard et al. (2002), we attribute them to the large-scale motion of the parent cloud. In the direction and at the distance of the source the parallaxes due to the Earth’s translation around the Sun and to the
differential galactic rotation are negligible in comparison with the measured absolute proper motions and are not considered here. 2.2. Relative Proper Motions We have used all four epochs to perform relative astrometry, taking as reference the position of the N component. Doing accurate relative astrometry from radio observations of sources like L1551 IRS 5 N and S is not easy because we are observing objects that are not pointlike, like a star, but relatively extended. As noted before, the 2 cm emission is most probably a combination of dust emission from the disks and of free-free emission from ionized ejecta. These sources have angular dimensions of order 0>1, and spatiotemporal brightness variations within them could produce the appearance of motion, affecting the astrometry. Indeed, this problem is evident in the two images shown in Figure 1, since the two sources in the 1983.89 and 1998.41 epochs show faint emission to the west and east,
18 01 42.6
18 01 42.6
1998.41
1983.89 N 42.4
DECLINATION (B1950)
DECLINATION (B1950)
42.4
42.2
S 42.0
42.2
N 42.0
S 41.8
41.8
41.6
41.6
04 28 40.27
40.26
40.25 40.24 40.23 40.22 RIGHT ASCENSION (B1950)
40.21
04 28 40.27
40.26
40.25 40.24 40.23 40.22 RIGHT ASCENSION (B1950)
40.21
Fig. 1.—VLA continuum images of L1551 IRS 5 at 2 cm, for the epochs 1983.89 (left) and 1998.41 (right). Contours are 3, 3, 4, 5, 6, 7, and 8 times the rms noise of the images (0.10 mJy beam1 for 1983.89 and 0.07 for 1998.41). The half-power contour of the beams are shown in the bottom left corner (see Table 1 for their sizes). The north (N) and south (S) components are indicated. Note the large absolute proper motion between the two epochs, whose values are listed in Table 2. The faint components to the east of the N and S components in the 1998.41 image are probably due to ejections of ionized gas (see text).
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Fig. 2.—Absolute proper motions in right ascension (left) and declination (right) for the north (solid line is the least-squares fit) and south (dashed line is the least-squares fit) components of L1551 IRS 5 as a function of time. The right ascension axis is given in seconds, while the declination axis is given in arcseconds.
respectively. This faint signal is probably free-free emission from recent ejecta. Also in Figure 1 we can see that the position angle of the vector that joins both sources seems to have changed slightly over the 14.5 yr period between the observations. Is this change statistically significant? To address this question quantitatively we analyzed the relative displacements in and of component S with respect to component N, as well as the projected separation and position angle of the relative displacement vector. The results of this relative astrometry are presented in Table 3. In Figure 3 we show the projected separation and position angle of the relative orbit as a function of time. Although the signal-to-noise ratio is modest, a trend is evident in both parameters since the separation shows a monotonic increase with time, while the position angle shows a monotonic decrease. A linear least-squares fit to the data gives rates of change for the separation and position angle of lsep ¼ þ2:1 0:7 mas yr1, and lpa ¼ 0:5 0:1 degree yr1 , respectively. The average projected separation for the four epochs is 306 17 mas, or 43 2 AU. The observed changes in separation and position angle result in a relative proper motion of 3:4 0:7 mas yr1 , implying a relative velocity in the plane of the sky of 2:3 0:5 km s1 . Comparable orbital proper motions have been detected in the subarcsecond binary system in L1527 by Loinard et al. (2002). A conservative lower limit to the total mass of the system can be derived since
TABLE 2 Absolute Proper Motions of L1551 IRS 5
Component
l (mas yr1)
l (mas yr1)
North............ South ............
+11.7 1.9 +14.7 1.6
20.3 2.5 22.2 2.8
ðM=M Þ ð1=2ÞðVobs =30 km s1 Þ2 ðrobs =AUÞ, where Vobs and robs are the projected relative velocity and separation. In terms of the observed parameters, the previous equation becomes ðM=M Þ 1:3ðl=mas yr1 Þ2 ðs=100 masÞ ðd=kpcÞ3 , where l is the relative proper motion, s is the angular separation between the members of the binary, and d is the distance to the source. Since mas yr 1, s ¼ 306 mas, and d ¼ 140 pc, we obtain M 0:1 M . We can make a less conservative estimate for the total mass of the system assuming that the plane of the orbit is parallel to the plane of the disks. This assumption is justified by the calculations of Bate et al. (2000), which suggest that the disks in a binary system will rapidly realign with the plane of the binary. For a binary system similar to L1551 IRS 5, this realignment is expected to take place on timescales of order 103–104 yr. Rodrı´guez et al. (1998) estimated the inclination angle of the disks to be 60 . Furthermore, since the major axes of the disks are approximately in the N-S direction, as is the separation vector, we can conclude that we are observing the binary in an epoch when the velocity in the plane of the sky is at its minimum. We therefore find that Vtrue ’ ðVobs = cosð60 ÞÞ ’ 5 km s1 . If we further assume that the orbit is circular, the observed separation will be
TABLE 3 Relative Astrometry in L1551 IRS 5a
Epoch
D (mas)
D (mas)
Separation (mas)
Position Angle (deg)
1983.89 ..... 1985.01 ..... 1995.57 ..... 1998.41 .....
+59 36 +40 18 +27 20 +13 24
293 25 283 15 320 13 320 18
299 26 285 15 320 13 319 18
191 7 188 4 185 4 182 4
a Relative position of the component S with respect to the component N.
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Fig. 3.—Projected separation (left) and position angle (right) of the relative orbit. The dashed line is a linear least-squares fit to the position angle data.
close to the true separation. We can then estimate the total mass and period of the system to be M ’ 1:2 M and P ’ 260 yr, respectively. Of the total mass of 1.2 M, about 0:09 M or 8% is estimated to be in the compact protoplanetary disks (Rodrı´guez et al. 1998). The derived total mass is similar, although somewhat smaller, to the average total masses of 1.7 M (Ghez et al. 1995) and 2.0 M (Woitas, Ko¨hler, & Leinert 2001) derived for groups of T Tauri binary systems from near-IR speckle measurements of their relative tangential velocities. On the average, one expects protobinaries to have smaller masses than formed binaries, since in the former important mass accretion is still occurring. If the two stars in L1551 IRS 5 have similar mass, in the main sequence the bolometric luminosity of the system would be 1 L, while at present it is 45 L (White et al. 2000). This result corroborates the statement that protostars obtain practically all their luminosity from accretion and not from nuclear reactions. The mass of the core where L1551 IRS 5 is embedded is 13 M (White et al. 2000), so the binary system seems to have accreted 10% of its core. If our model for the orbit is correct, we expect the position angle to continue decreasing over the years with the rate of change increasing until a maximum is reached in 60 yr, when the binary will be aligned in the E-W direction. 3. CONCLUSIONS
Our main conclusions can be summarized as follows. 1. We detected absolute proper motions in the radio binary L1551 IRS 5. These absolute proper motions are
very similar in magnitude and direction to those of T Tauri stars in the same region and are attributed to the large-scale motion of the parent molecular complex. 2. We also detect orbital motions in the binary. With the additional information provided by our knowledge of the binary disk system, we estimate the total mass and period of the binary system to be 1.2 M and 260 yr, respectively. 3. It has been possible to determine the mass of T Tauri stars by near-IR speckle measurements of their relative tangential velocities (Ghez et al. 1995; Woitas et al. 2001) or by measurement of the rotation of their circumstellar disks (Simon, Dutrey, & Guilloteau 2000). However, the type of observations presented here and in Loinard (2002) and Loinard et al. (2002) will allow the determination of the masses of even younger, extremely obscured stars, for which no reliable technique was available until now. It is expected that, on the average, protobinaries will have smaller masses than formed binaries. With the advent of centimeter- and millimeter-wave interferometers of high sensitivity and astrometrical capabilities, like the EVLA and ALMA, it will be possible to follow not only the relative but also the absolute orbital motions of many protobinary systems. L. F. R. is grateful to the support of CONACyT, Me´xico and DGAPA, UNAM. S. C. acknowledges support from CONACyT grant 33933-E. J. C. acknowledges support from CONACyT grants 34566-E and 36572-E. J. M. T. is partially supported by DGESIC grant PB98-0670-C02.
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