A bright star is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star.
 |
| Wide-field image from DSS2. U Camelopardalis is the bright orange star at the center of the image. Credit: DSS2 / SkyMap.org |
U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds.
Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the centre of the image. Its brightness, however, is enough to overwhelm the capability of Hubble’s Advanced Camera for Surveys making the star look much bigger than it really is.
The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable (see for example Hubble’s images of
Eta Carinae), the shell of gas expelled from U Cam is almost perfectly spherical.
In a Letter published in 1999 on
Astronomy & Astrophysics, Lindqvist, Olofsson and other colleagues from Sweden, France and Spain produced a list of physical parameters about U Camelopardalis. This is the abstract from the paper:
We report IRAM Plateau de Bure interferometer observations of the carbon star U Cam in the CO(J = 1 → 0) and CO(J = 2 → 1) lines. The remarkable images show that U Cam is surrounded by a geometrically thin, ∼1016 cm, shell of gas at a distance of ∼6×1016 cm from the star, that expands with a velocity of ∼23 km s-1. The estimated mass of the shell is low, ∼10-3 M⊙. In addition, we detect emission that peaks at the stellar position. From this we estimate a present mass loss rate and gas expansion velocity of ∼2.5×10-7 M⊙ yr-1 and 12 km s-1, respectively. One possible explanation to the structure of the circumstellar medium is that the shell was produced during a very short period, ∼150 yr, of high mass loss rate, ∼10-5 M⊙ yr-1, about 800 yr ago. U Cam may fit into the scenario where a helium-shell flash modulates the mass loss rate on short times scales.
More than ten years later, another research paper from H. Olofsson and other Sweden astronomers used the Hubble's ACS images to present updated information on U Camelopardalis and its expanding shell. Here is the abstract from this more recent paper:
Context. Overall spherically symmetric, geometrically thin gas and dust shells have been found around a handful of asymptotic giant branch (AGB) carbon stars. Their dynamical ages lie in the range of 103 to 104 years. A tentative explanation for their existence is that they have formed as a consequence of mass-loss-rate modulations during a He-shell flash. Aims. The detached shells carry information on their formation process, as well as on the small-scale structure of the circumstellar medium around AGB stars due to the absence of significant line-of-sight confusion.
Methods. The youngest detached shells, those around the carbon stars R Scl and U Cam, are studied here in great detail in scattered stellar light with the Advanced Survey Camera on the Hubble Space Telescope. Quantitative results are derived assuming optically thin dust scattering.
Results. The detached dust shells around R Scl and U Cam are found to be consistent with an overall spherical symmetry. They have radii of 19”.2 (corresponding to a linear size of 8×1016 cm) and 7”.7 (5×1016 cm), widths of 1”.2 (5×1015 cm) and 0”.6 (4×1015 cm), and dust masses of 3×10-6 and 3×10-7 M⊙, respectively. The dynamical ages of the R Scl and U Cam shells are estimated to be 1700 and 700 yr, respectively, and the shell widths correspond to time scales of 100 and 50 yr, respectively. Small-scale structure in the form of less than arcsec-sized clumps is clearly seen in the images of the R Scl shell. Average clump dust masses are estimated to be about 2×10-9 M⊙. Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (≤ 1“ for U Cam and ≈2“ for R Scl).
Conclusions. The results are consistent with the interpretation of geometrically thin gas and dust shells formed by a mass-loss eruption during a He-shell flash, and where interaction with a previous wind plays a role as well. The mass loss responsible for the shells must have been remarkably isotropic, and, if wind interaction plays a role, this also applies to the mass loss prior to the eruption. Clumpy structure is present in the R Scl shell, possibly as a consequence of the mass loss itself, but more likely as a consequence of instabilities in the expanding shell.
 |
| U Camelopardalis imaged with the f606 filter of the Hubble's ACS instrument. Credit: H. Olofsson, M. Maercker, K. Eriksson, B. Gustafsson, and F. Schöier (2010) |
 |
| U Camelopardalis imaged with the f814 filter of the Hubble's ACS instrument. Credit: H. Olofsson, M. Maercker, K. Eriksson, B. Gustafsson, and F. Schöier (2010) |
 |
| U Camelopardalis imaged with the High Resolution Channel of the Hubble's Advanced Camera for Surveys. Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory) |
 |
| Enlarged detail of the previous image. Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory) |
Sources:
- V* U Cam – Semi-regular pulsating Star, SIMBAD Database
- Red Giant Blows a Bubble, Hubble image release, July 2, 2012
- M. Lindqvist et al., The young detached CO shell around U Camelopardalis, Astronomy and Astrophysics, v.351, p.L1-L4 (1999)
- H. Olofsson et al., High-resolution HST/ACS images of detached shells around carbon stars, arXiv:1003.0362v1, March 1, 2010
