did carinae's weigelt blobs originate circa 1941? - IOPscience

The Astronomical Journal, 127:1052–1058, 2004 February # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.

DID
CARINAE’S WEIGELT BLOBS ORIGINATE CIRCA 1941? Bryan N. Dorland1 Astrometry Department, US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392

Douglas G. Currie Department of Physics, University of Maryland, College Park, MD 20742-4111

and Arsen R. Hajian Astrometry Department, US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392 Received 2003 March 6; accepted 2003 October 22

ABSTRACT We report astrometric measurements of
Carinae’s Weigelt blobs C and D (also known as speckle objects C and D) derived from observations with the Hubble Space Telescope Wide Field Planetary Camera 2. Using three epochs of data with a temporal baseline of 5.9 yr, we measure blob D’s proper motion relative to the central star to be 4.4
1.4 mas yr1, implying a date of origin of 1934.1þ16:0 31:7 . Similarly, using two epochs with a 1.7 yr baseline, we measure blob C’s relative proper motion to be 3.8
5.6 mas yr1, indicating a probability of ejection after 1910 of 60%; we view this as confirming the more accurate blob D results (assuming coevality), with a combined post-1910 ejection indicated at a 90% confidence level. The blob D date roughly coincides with the sudden brightening of the central star observed in 1941 and provides a possible explanation for the onset of certain narrow spectral features, associated with the Weigelt blobs, that have been observed only since the mid-1940s. We propose that the ejection of Weigelt blobs C and D was related to this 1941 event. Key words: astrometry — ISM: jets and outflows — stars: individual (
Carinae) — stars: variables: other

1. INTRODUCTION

of the material in the bipolar nebula surrounding the star (Currie et al. 1996), commonly called the ‘‘Homunculus.’’ Another notable eruption occurred around 1890, the so-called Lesser Eruption (Walborn & Lillier 1977; Humphreys, Davidson, & Smith 1999). Other sudden brightening events have also been observed; we discuss these later in the paper. These quasi-periodic eruptions typically eject a large amount of mass into the interstellar medium and are responsible for the generation of a unique debris field about the central star, populated with a variety of features (in addition to the papers already cited, see Thackeray 1949, Gaviola 1950, Ringuelet 1958, Gehrz & Ney 1972, Walborn 1976, Meaburn, Wolstencroft, & Walsh 1987, Smith & Gehrz 1998, and Morse et al. 2001 for descriptions and astrometric measurements of the debris surrounding
Car). It would not be surprising if the Weigelt blobs were ejected from the star as the result of one of these events. One of the goals of this paper is to ask whether the ejection of the Weigelt blobs is related to any of the historical eruptions.

One of the most massive and luminous stars in our galaxy,
Carinae is also generally recognized as one of the most enigmatic. One of its many mysteries involves the origin and nature of the three close-in ejecta, originally called the ‘‘speckle objects’’ but now known as the ‘‘Weigelt blobs,’’ shown in Figure 1. These three objects (commonly referred to as blobs B, C, and D) were first discovered using speckle techniques as described by Weigelt & Ebersberger (1986) and Hofmann & Weigelt (1988). The separation between the blobs and the central star has been successfully resolved using the Hubble Space Telescope (HST ) Wide Field Planetary Camera 2 (WFPC2) (Morse et al. 1998). Combined, these observations indicate that the three objects all lie within 300 milliarcseconds (mas) of the central star and are less than 50 mas in diameter. Adopting a distance of 2.2 kpc (Allen & Hillier 1993), these correspond to a separation of 660 AU and a maximum diameter of 110 AU. These clumps of ejecta are thought to be compact knots of gas ejected from the central star at some point in the past (Davidson et al. 1995). The determination of precisely when this ejection took place for blobs C and D is the subject of this paper. The record of
Car is a history of a series of eruptions, resulting in sudden increases and diminutions in brightness. Most notable in the last 200 years of continuous observation is the Great Eruption of 1842, during which
Car briefly became one of the brightest stars in the sky and then, over the course of a few years, became so dim it was invisible to the naked eye. The Great Eruption is responsible for the ejection

2. PRIOR MEASUREMENTS OF THE WEIGELT BLOB PROPER MOTIONS AND ORIGIN DATES In order to determine the date of origin for the blobs, measurements of the blob–star separation from at least two epochs are necessary. Unfortunately, the very bright central source and highly structured background conspire to make astrometric measurements of the position and proper motion of the nearby blobs extremely difficult. In our analysis, we found that these factors severely complicate accurate astrometry for blob B, the closest to the central star. In this paper we restrict our analysis to blobs C and D. The only published proper-motion measurement available is that made by Weigelt et al. (1995, hereafter W95). In W95, the authors combined speckle position measurements (epoch 1)

1 Also Department of Physics, University of Maryland, College Park, MD 20742-4111.

1052

WEIGELT BLOBS

1053

N Blob D

Reference Field Stars

E Homunculus nebula Blob B Blob C

Central Star

N Central Star region (magnified) E 10 arcsec

300 mas

Fig. 1.—Left: F631N PC1 image. The Homunculus Nebula is noted, and the central-star region, which contains the Weigelt blobs, is indicated with a box. Right: The magnified central-star region. Positions of the central star and the three Weigelt blobs are indicated.

with HST Faint Object Camera measurements (epoch 2). The two epochs provide a temporal baseline of approximately 8 years. The relative separation measurements from W95 result in proper-motion values of 3.95 and 3.85 mas yr1 for blobs C and D, respectively.2 Assuming radial, ballistic motion, the W95 values yield ejection dates of 1940 (blob C) and 1930 (blob D). The authors caution that the measurements may be in error, as the observations were made at different wavelengths with different instruments, and conclude that ‘‘the simplest interpretation is that components C and D were thrown out from the star at some time between 1880 and 1930’’ (p. 15). How this conclusion follows from the data is not self-evident. A reference to a second set of unpublished proper-motion measurements also exists. In Davidson et al. (1997, hereafter D97), the authors cite an unpublished result3 for a mean proper motion for blobs C and D of 2.4
0.8 mas yr1. Using this value for the proper motion, they obtain a value for the date of ejection for C and D of 1910,
40 years. The authors also state that ‘‘[a]n obvious and appealing idea is that CD were ejected in the Lesser Eruption of
Car observed around 1890’’ (p. 343). Since publication in D97, this 1890 date has become the ‘‘canonical’’ date of origin for the Weigelt blobs (see Humphreys 2002),4 despite the fact that the W95 results indicate a much later date (1935). 3. ASTROMETRIC MEASUREMENTS USING WFPC2 DATA In order to accurately resolve the apparent disagreement between the W95 results and the canonical ejection date, we decided to independently determine an ejection date for blobs C and D. To this end, we selected a subset of all publicly

available HST WFPC2 observations of
Car for astrometric analysis. 3.1. Data Selection We selected data for each blob based on the following criteria: First, two epochs of data had to exist for a given WFPC2 filter. This was motivated by a desire to avoid spurious motions that might arise when using different-filter data for the two epochs. This can occur, for example, when different line emissions originate in different physical locations in the blob as a result of local astrophysical conditions. Second, all the observations had to have been made with the PC1 CCD in order to take advantage of its enhanced spatial resolution. Third, the image in the vicinity of both the blobs and the central star had to be unsaturated. Some images we considered were rejected because the single pixel containing the photocenter of the central star appeared to be saturated. The seven images satisfying the criteria are listed in Table 1. Of the seven, four were taken with the F658N filter and three with F631N filter. Three observations date from 1995, two from 1997, and two from 2001. The 1995 and 1997 observations give us two blob C epochs with a temporal baseline of 1.7 yr, and the 1995, 1997, and 2001 observations of D give us three epochs with a 5.9 yr baseline. ‘‘On the fly’’ calibration was applied to all data by the HST archive system during retrieval. Distortion correction was performed using the ‘‘Drizzle’’ process (Fruchter & Hook 2002) within IRAF/STSDAS using the appropriate Trauger distortion coefficients (Holtzman et al. 1995). Cosmic-ray subtraction was not performed, because of the very short integration times and very small image fields involved. 4. ANALYSIS

2

W95 does not provide quantitative error values for the measurements. D97 cites as reference for the blob proper motion both W95 and a second paper listed as ‘‘in press.’’ These references are problematic in that (1) the W95 results for proper motion differ significantly from the D97 value, and (2) the second reference was apparently never published. 4 Available at http://www.astro.washington.edu/balick/eta_conf/papers/ humphreys.History.pdf. 3

4.1. Relative Astrometry Methodology After initial processing was completed, further processing was performed in order to obtain the relative astrometric separation between the blobs and the central star. Median images were created by passing a 5  5 kernel over each source image. The median images were then subtracted from the source

1054

DORLAND, CURRIE, & HAJIAN

Vol. 127

TABLE 1 WFPC2 Data Used for Astrometric Analysis

Image ID

Observation Date

Filter

Exp. Time (s)

P.A. V3 Angle (deg)

Blob

95_631a...... 95_658a...... 95_658b...... 97_631a...... 97_631b...... 01_658a...... 01_658b......

1995.723 1995.441 1995.441 1997.435 1997.435 2001.424 2001.424

F631N F658N F658N F631N F631N F658N F658N

0.5 0.11 0.11 1.2 1.2 0.11 0.11

19.0 300.0 300.0 286.4 286.4 300.0 300.0

C, D D D C, D C, D D D

images, producing a set of median-subtracted images with significantly suppressed background. Using a 5  5 grid search, a least-squares three-parameter fit of a two-dimensional Gaussian with fixed width (FWHM = 1.6 pixels) was used to determine the positions of the central star and the blobs in each median-subtracted image. Centroid positions of the central star and blobs were then used to solve for relative separation and position angle in each image. Figure 2 graphically displays the results for two of the analyzed images. 4.2. Relative Astrometry Results The results from the median subtraction method are shown in Table 2, including the measured separation between the central star and the blobs, the predicted error for the measurement, and the resultant position angle. We successfully measured separations for blob D in both filter bands, but we were only able to obtain unambiguous results for blob C in the F631N images. We discuss the method for deriving the errors in x 4.3. For blob D, the resultant weighted mean separation and position angle using the median subtraction method are 



d1995:54;D ¼ 268:6
5:8 mas;

PA1995:54;D ¼ 338 : 1
0 :8

d1997:44;D ¼ 275:7
4:9 mas;

ðepoch 1Þ;   PA1997:44;D ¼ 335 : 8
0 :5

d2001:42;D ¼ 294:2
6:4 mas;

ðepoch 2Þ;  ¼ 339 : 1
1 :1

PA2001:42;D



ðepoch 3Þ:

For blob C, the results are d1995:72;C ¼ 215:3
8:3 mas; d1997:44;C ¼ 222:2
4:9 mas;



PA1997:44;C

The position angle results are consistent with linear, radial, ballistic motion for blobs C and D; no evidence of azimuthal motion was detected. Overall weighted position angle measurements for C and D are PAC = 300 . 6
0 . 6 and PAD = 336 . 8
0 . 4. These values are in good agreement with the measurements of PAC = 300 . 25 and PAD = 335 . 8 from W95. 4.3. Error Analysis We performed an extensive error analysis on the images listed in Table 1 in order to validate the median subtraction method and quantify the resultant errors. This error analysis consisted of adding simulated blobs (modeled as a twodimensional Gaussian using the parameters appropriate to the observed blob in the image) at seven different field positions in each image. The simulation positions were defined by rotating the separation vector about the central star’s position in increments of 45, spanning the range 45 –315 . This process effectively creates an ‘‘annulus’’ of seven field positions surrounding the central star at a radius equal to the central star– blob separation, as shown in Figure 3.

Central Star

E

N

ðepoch 1Þ;  ¼ 299 :8
0 : 7 

ðepoch 2Þ:

Central Star

Blob D



PA1995:72;C ¼ 301 :9
0 : 9

E

Blob D

N

Fig. 2.—Sample F658N images near the central star. Left, 1995 image; right, 2001 image. The positions of the central star and the blob D centroid are marked with crosses. The pixels are 45.5 mas across.

No. 2, 2004

WEIGELT BLOBS

1055

TABLE 2 Relative Separation and Position Angle Measurements Blob C

Blob D

Image ID

Separation (mas)

P.A. (deg)

95_631a......... 95_658a......... 95_658b......... 97_631a......... 97_631b......... 01_658a......... 01_658b.........

215.3
8.3 ... ... 224.8
6.6 218.5
6.6 ... ...

301.9
0.9 ... ... 299.6
0.9 300.0
1.1 ... ...

Each simulated blob image was then processed as described in x 4.1. The centroid was determined for each position around the annulus, and separation and angle with respect to the central star were calculated. A few of the regions around these positions (