22 research outputs found
Detection of ionized gas in the globular cluster 47 Tucanae
We report the detection of ionized intracluster gas in the globular cluster
47 Tucanae. Pulsars in this cluster with a negative period derivative, which
must lie in the distant half of the cluster, have significantly higher measured
integrated electron column densities than the pulsars with a positive period
derivative. We derive the plasma density within the central few pc of the
cluster using two different methods which yield consistent values. Our best
estimate of n_e = (0.067+-0.015)/cm^3 is about 100 times the free electron
density of the ISM in the vicinity of 47 Tucanae, and the ionized gas is
probably the dominant component of the intracluster medium.Comment: 5 pages, 3 included figures, accepted for publication by ApJ Letter
Stellar mass loss and the Intra-Cluster Medium in Galactic globular clusters: a deep radio survey for HI and OH
We present the results of a survey, the deepest to date, for HI emission at
21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720 MHz) in the
direction towards the Galactic globular clusters M 15, M 2, NGC 6934, NGC 7006
and Pal 13. The aim is to measure the amount of hydrogen in the intra-cluster
medium (ICM), and to find OH masers in the circumstellar envelopes of globular
cluster red giants. We present a tentative detection of 0.3 Msun of neutral
hydrogen in M 15 and possible detections of neutral hydrogen in M 2 and Pal 13.
We derive upper limits to the neutral hydrogen content of NGC 6934 and NGC
7006. No OH emission is detected. We also present deep HI data of the northern
tip of the Magellanic Stream behind Pal 13.Comment: Accepted for publication in MNRAS. 7 page
A Spitzer search for cold dust within globular clusters
Globular cluster stars evolving off the main sequence are known to lose mass,
and it is expected that some of the lost material should remain within the
cluster as an intracluster medium (ICM). Most attempts to detect such an ICM
have been unsuccessful. The Multiband Imaging Photometer for Spitzer on the
Spitzer Space Telescope was used to observe eight Galactic globular clusters in
an attempt to detect the thermal emission from ICM dust. Most clusters do not
have significant detections at 70 microns; one cluster, NGC 6341, has tentative
evidence for the presence of dust, but 90 micron observations do not confirm
the detection. Individual 70 micron point sources which appear in several of
the cluster images are likely to be background galaxies. The inferred dust mass
and upper limits are < 4e-4 solar masses, well below expectations for cluster
dust production from mass loss in red and asymptotic giant branch stars. This
implies that either globular cluster dust production is less efficient, or that
ICM removal or dust destruction is more efficient, than previously believed. We
explore several possibilities for ICM removal and conclude that present data do
not yet permit us to distinguish between them.Comment: AJ in press; 30 pages with 8 figure
ISOCAM observations of Galactic Globular Clusters: mass loss along the Red Giant Branch
Deep images in the 10 micron spectral region have been obtained for five
massive Galactic globular clusters, NGC 104 (=47 Tuc), NGC 362, NGC 5139 (omega
Cen), NGC 6388, NGC 7078 (=M15) and NGC 6715 (=M54) in the Sagittarius Dwarf
Spheroidal using ISOCAM in 1997. A significant sample of bright giants have an
ISOCAM counterpart but only < 20% of these have a strong mid-IR excess
indicative of dusty circumstellar envelopes. From a combined physical and
statistical analysis we derive mass loss rates and frequency. We find that i)
significant mass loss occurs only at the very end of the Red Giant Branch
evolutionary stage and is episodic, ii) the modulation timescales must be
greater than a few decades and less than a million years, and iii) mass loss
occurrence does not show a crucial dependence on the cluster metallicity.Comment: 26 pages, 9 figure
Far infra-red emission from NGC 7078: First detection of intra-cluster dust in a globular cluster
Improved data analysis of far infrared imaging data of the
globular cluster NGC 7078 obtained with the ISO instrument
ISOPHOT at 60 μm, 70 μm and 90 μm has detected the thermal
emission from dust in its core, the first secure detection of
intra-cluster dust in a globular cluster. The amount of dust is
broadly consistent with mass-loss from evolved, metal-deficient
stars in NGC 7078 in the time since it last crossed the Galactic
plane.