Herschel

Mission status in progettazione

Background

The Herschel ESA space telescope was launched on May 14th, 2009, from the French site in Korou. It carried on board three instruments: PACS, for photometric and spectroscopic observations in the range ~50 – 200 micron; SPIRE, like PACS but in the range 200 - 500 micron; and HIFI, that observed spectroscopically almost the entire range covered by Herschel.

This region of the electromagnetic spectrum cannot be observed from ground, because of the water vapour in the atmosphere that absorbs the radiation coming from space. This region, however, is important for astronomy because the interstellar dust, at a temperature few degrees above the absolute zero (-273 °C), emits the bulk of its radiation at these wavelengths. The interstellar dust, made of grains with size less than millimetre and containing mainly carbon and silicon, plays an important role in star formation. The dust makes it possible to keep low the temperature of the gas condensations that form in the interior of molecular clouds, giant structures filamentary in shape where stars form.

The birth of a star takes very long times, of the order of 100,000 years for the stars more massive than our Sun, up to millions years for lower mass stars. As a consequence, to reconstruct the star forming process, the only possibility we have is to observe a large number of forming stars, each one in its own evolutionary stage. After collecting the observations that catch the stars in different stages of their formation we can arrive to a complete picture of the process.

This is what we have done with the three Herschel instruments. 

In detail

IAPS, our institute, contributed to build all the instruments, the only institute all over the world to do that, and for this reason we could be members of many observing programs in the field of star formation. The two most important are

    1. the observation of the closest star forming regions, within 500 parsec from us. This program was named HGBS, Herschel Gould Belt Survey. The name is after Gould, the US astronomer who discovered, in the second half of 1800,  that the main nearby molecular clouds lay on a belt in the sky, even if the belt had already be noted few decades before by Herschel, the English astronomer to whom the space telescope was dedicated. The origin of this belt is still debated. The regions, found in the constellations of Lupus, Taurus, Ophiuchus, Perseus, Serpens and Orions, contain almost only low-mass stars that, once formed, will have masses below 8-10 solar masses. The only exception is Orion where, on the contrary, also high-mass stars form, with more than 10 solar masses;

    2. the observation of the most important regions of high-mass star formation, within 3,000 parsec from us. The name is HOBYS, or Herschel imaging survey of OB Young Stellar objects. Letters O and B are used for the most massive stars.

Both programs used all the 6 photometric bands (70, 100 e 160 micron for PACS, 250, 350 e 500 micron for SPIRE). Inside the different regions we identified the “cores”, the compact condensations of gas and dust that may form stars. Thanks to Herschel we could tell which cores are only transient structures and which ones are, likely, collapsing under their self-gravity, ending in stars in the future.

HGBS and HOBYS programs have already achieved important results like:

    1. the importance of the filamentary structures that characterise the shape of the molecular clouds. Stars form mainly in the filaments;

    2. the number of forming stars per mass unit looks similar to that of already formed stars in the neighbourhood of the Sun; then, we conclude that the mass of a star is somehow fixed by the initial conditions present when the star formation process begins;

    3. the observation of forming stars in a particular phase of their formation, never observed before. In this phase the collapsing core reaches for the first time the hydrostatic equilibrium. This phase was foreseen by star formation theory at the end of the 60’s last century, but only in the last years astronomers are reasonably confident to have catched a few cores in this very short stage, a few of hundreds/thousands years. And this success is due to  Herschel.

Herschel data analysis for HGBS and HOBYS programs is still on going, and our team is the main investigator of some of the regions. Among them, Orion stands out, the most important star formation site of the HGBS program.

Another important program is Hi-Gal,  Herschel Infrared Galactic Plane Survey. This program was led by S. Molinari of IAPS. Hi-Gal with the aim of quantifying Galaxy-wide the relationship between the physical agents responsible for the onset and the regulation of star formation in a spiral galaxy and the resulting rate and efficiency of star formation.

Hi-GAL has been playing a major role in this scenario, thanks to the unprecedented quality of Herschel observations, in terms of resolution, sensitivity and dynamical range, in a crucial spectral domain for studying emission from cold dust in interstellar clouds. 

Hi-GAL data processing was carried out at IAPS. An effort to build a dedicated reduction pipeline and a final map making engine was made in synergy among IAPS, and the universities of Rome “La Sapienza” and “Tor Vergata”, producing the UNIMAP algorithm. As a consequence of this, the Rome pole became a reference for production of best-quality Herschel photometric maps.

Furthermore, two codes were developed to perform compact source and filament detection and extraction. This allowed the Hi-GAL team to produce two catalogs of capital importance and unprecedented level of completeness. First, the photometric catalog of the compact sources, namely the demographics of Galactic clumps and cluster progenitors, present in the inner Galaxy. Second, the catalog of outstanding and pervasive filamentary structures across the entire Galactic plane, which represents the most complete census so far of these pivotal structures that bring diffuse gas and dust in the Milky Way to form the nurseries of new generations of stars. Hi-GAL is therefore providing a science legacy for decades to come with unprecedented potential for systematic and serendipitous science in a wide range of astronomical fields.

The achieved level of excellence and position of primacy in the research on Galactic star formation is now kept by our research group at IAPS by bringing to a common forum the major new-generation surveys of the Galactic Plane from 1 μm to the radio, both in thermal continuum and in atomic and molecular lines. Combination of near-Infrared (IR) ground surveys data, mid-IR and far-IR dust continuum obtained by Herschel, Spitzer, and WISE satellites, with radio free-free continuum and gas-tracing atomic and molecular spectroscopy from ground-based observatories, which allows to compile the first complete census of sites of ongoing and potential star formation in the Milky Way. 

Team

Stefano Pezzuto - IAPS (Italian responsible for per HGBS e HOBYS)

Milena Benedettini - IAPS (co-responsisble)

Sergio Molinari - IAPS (PI Hi-Gal)

Anna Maria di Giorgio - IAPS

Davide Elia - IAPS

Scige Liù

Eugenio Schisano – IAPS

Kazi Rygl – IRA

Danae Polychroni – Oss. Trieste

Diego Turrini – Oss. Torino

Alessio Traficante IAPS