University of Alicante

Alicante Clusters

Alicante: a family of star clusters in our Galaxy

This is work carried out by the research group known as:

Stellar Astrophysics at the University of Alicante

It belongs to one of the research lines we pursue:

The study of stellar populations in young open clusters, which are more or less numerous groups of gravitationally bound stars lying at the same distance from us and sharing the same age. These objects let us tackle several current problems in astrophysics: Galactic structure and the evolution of massive stars.

Astrophysical data for the clusters

We have discovered new clusters in our Galaxy which we have named Alicante. So far we have a family of 12 such clusters. Each of them has distinct characteristics and a different population, which allows us to study a range of topics.

A few are worth singling out: Alicante 1, which contains a binary system, MY Cam, a candidate to merge into a single object (named one of the most interesting objects in our Galaxy by the BBC programme Sky at Night), and others such as Alicante 10 and Alicante 8, which are among the most massive clusters in our Galaxy (Alicante 8 was featured in Steven Spielberg’s series Falling Skies).

When an astronomical object is discovered — a star cluster, in this case — it is named as follows: if a single person made the discovery, their surname is used; if more than one person was involved, the name of the institution they work at is used instead. Where there are several objects, they are numbered in sequence: Alicante 1, Alicante 2, and so on. Among the families named this way are the traditionally known Berkeley clusters in the USA and Bochum in Germany, now joined by the Alicante clusters — the first Spanish institution to have a family of star clusters to its name.

Our line of work is observational astrophysics, so we regularly write or collaborate on observing-time proposals that competitively win dozens of nights on the most sought-after telescopes in the world, through their time allocation committees or the OPTICON programme (Isaac Newton Group, GRANTECAN, Very Large Telescope, New Technology Telescope, Australian Astronomical Observatory, and others). This has made us regular users of these magnificent facilities around the world for many years now.

Alicante Clusters

Alicante 1

The paper was published in 2008 in the journal Astronomy and Astrophysics: Negueruela & Marco, 2008, A&A, 492, 441.

This work, part of our research on Galactic structure, studied the only stellar association, Cam OB3, that was thought to belong to the outermost arm of the Milky Way (the Cygnus arm). We concluded that this association of stars does indeed belong to that arm and, in addition, we found the star cluster that belongs to it: Alicante 1. They lie at a distance of 4.0 ± 0.4 kpc, which gives us an estimate of the distance to that arm in that direction.

It lies in the constellation of Camelopardalis. Its coordinates are: R.A. 03h 59m 18.3s; Dec. +57° 14’ 13.7’’ (J2000).

Alicante 1 — False-colour image taken by the authors with the ALFOSC instrument on the Nordic Optical Telescope at the Roque de los Muchachos Observatory, La Palma (NOT/ALFOSC)

Alicante 2

The paper was published in 2003 in the journal Astronomy and Astrophysics: Negueruela, I. 2003, A&A, 408, 689.

This work identifies the three brightest stars belonging to a very compact young cluster within the Cassiopeia OB5 (Cas OB5) stellar association, which we later named Alicante 2. One of them, BD +62°2296A, is a supergiant of spectral type B2.5Ia. The nearest to it, BD +62°2296B, is a Wolf-Rayet star of type WN4, and the third, BD +62°2296C, is a giant star of spectral type B0III. They are individual stars, forming neither a binary nor a triple system. Such a concentration of very massive stars, not belonging to a massive cluster in which a large number of slightly less massive stars ought to appear as well, is highly interesting for the study of the formation and evolution of massive stars.

It lies in the constellation of Cassiopeia and its coordinates are: R.A. 23h 47m 20.4s; Dec. +63° 13’ 12’’ (J2000).

Alicante 5

The image of the cluster was obtained with the NTT (La Silla Observatory, Chile) at visible wavelengths.

The false-colour composite was obtained with the Spitzer space telescope and shows that the visible stars are part of a much larger population of extremely young stars, which is starting to emerge from the cloud in which it formed.

Alicante 6

This image, obtained with the Spitzer space telescope, shows in the mid-infrared a bubble of interstellar dust known as N37. Inside it lies an extremely young cluster that has not yet fully emerged from the cloud: Alicante 6.

Our recent work shows that star formation in this area was probably triggered by a very luminous star of spectral type O7 II, located in the lower right corner of the image (being a very hot star, it is barely visible in the infrared).

Alicante 6, a young cluster. False-colour composite of three images

Alicante 7, 8, 9, 10

Alicante 8, a three-band composite taken in the near-infrared from the UKIDSS data catalogue

These clusters are among the most massive in our Galaxy (their stellar content exceeds 10,000 solar masses). They lie at a distance of 6 kpc and are grouped where the bar meets the spiral arm known as Scutum-Crux. They are very far away and in a region of very high extinction, so we can only detect and study their brightest members, the red supergiants. These objects radiate most strongly in the infrared, so they have to be observed with detectors that capture light in that wavelength range. If we try to observe them at visible wavelengths nothing shows up at all, as though they did not exist.

Some papers on these clusters in the journal Astronomy and Astrophysics:

Alicante 11, 12

The paper in which they are published, in the journal Monthly Notices of the Royal Astronomical Society:

Marco, A. & Negueruela, I. 2016, MNRAS, 459, 880-901

This work belongs to our research on Galactic structure and star formation. We were studying a star-forming region containing the open cluster Stock 8, in order to investigate the extent of the Auriga OB2 stellar association and the possible star-formation mechanisms at work in the area. As a result, Stock 8’s membership of that association has been ruled out, placing it much closer, at 2.8 kpc, in the Perseus arm. In the Stock 8 region we found two new clusters lying at the same distance as it: Alicante 11 and Alicante 12.

Image taken in Aladin from the WISE mid-infrared catalogue. The blue dots are member stars of the Stock 8 cluster, and the yellow and green ones belong to the clusters Alicante 11 and Alicante 12.